9.0 Chapter Introduction
9.0.1 Chapter IntroductionPage 1:
Enhanced Interior Gateway Routing Protocol (EIGRP) is a distance vector, classless routing protocol that was released in 1992 with IOS 9.21. As its name suggests, EIGRP is an enhancement of Cisco IGRP (Interior Gateway Routing Protocol). Both are Cisco proprietary protocols and only operate on Cisco routers.
The main purpose in Cisco's development of EIGRP was to create a classless version of IGRP. EIGRP includes several features that are not commonly found in other distance vector routing protocols like RIP (RIPv1 and RIPv2) and IGRP. These features include:
- Reliable Transport Protocol (RTP)
- Bounded Updates
- Diffusing Update Algorithm (DUAL)
- Establishing Adjacencies
- Neighbor and Topology Tables
Note: The term hybrid routing protocol is sometimes used to define EIGRP. However, this term is misleading because EIGRP is not a hybrid between distance vector and link-state routing protocols - it is solely a distance vector routing protocol. Therefore, Cisco is no longer using this term to refer to EIGRP.
In this chapter, you will learn how to configure EIGRP and verify your EIGRP configuration with new show commands. You will also learn the formula used by EIGRP to calculate this composite metric.
Unique to EIGRP is its Reliable Transport Protocol (RTP) which provides reliable and unreliable delivery of EIGRP packets. In addition, EIGRP establishes relationships with directly connected routers that are also enabled for EIGRP. Neighbor relationships are used to track the status of these neighbors. RTP and the tracking of neighbor adjacencies set the stage for the EIGRP workhorse, the Diffusing Update Algorithm (DUAL).
As the computational engine that drives EIGRP, DUAL resides at the center of the routing protocol, guaranteeing loop-free paths and backup paths throughout the routing domain. You will learn exactly how DUAL selects a route to install in the routing table and what DUAL does with potential backup routes.
Like RIPv2, EIGRP can operate with classful or classless routing behavior. You will learn how to disable automatic summarization and then how to manually summarize networks to reduce the size of routing tables. Finally, you will learn how to use default routing with EIGRP.
9.0.1 - Chapter Introduction
The diagram depicts the Enhanced Interior Gateway Routing Protocol (EIGRP), a distance-vector, classless routing protocol developed by Cisco.
The chapter objectives are listed:
- Describe the background and history of EIGRP.
- Describe the features and operation of EIGRP.
- Examine the basic EIGRP configuration commands and identify their purposes.
- Calculate the composite metric used by EIGRP.
- Describe the concepts and operation of DUAL.
- Describe the uses of additional configuration commands in EIGRP.
The diagram depicts the Enhanced Interior Gateway Routing Protocol (EIGRP), a distance-vector, classless routing protocol developed by Cisco.
The chapter objectives are listed:
- Describe the background and history of EIGRP.
- Describe the features and operation of EIGRP.
- Examine the basic EIGRP configuration commands and identify their purposes.
- Calculate the composite metric used by EIGRP.
- Describe the concepts and operation of DUAL.
- Describe the uses of additional configuration commands in EIGRP.
9.1 Introduction to EIGRP
9.1.1 EIGRP: An Enhanced Distance Vector Routing ProtocolPage 1:
Although EIGRP is described as an enhanced distance vector routing protocol, it is still a distance vector routing protocol. This can sometimes be a source of confusion. In order to appreciate enhancements of EIGRP and eliminate any confusion, we must first look at its predecessor, IGRP.
Roots of EIGRP: IGRP
Cisco developed the proprietary IGRP in 1985, in response to some of the limitations of RIPv1, including the use of the hop count metric and the maximum network size of 15 hops.
Instead of hop count, both IGRP and EIGRP use metrics composed of bandwidth, delay, reliability, and load. By default, both routing protocols use only bandwidth and delay. However, because IGRP is a classful routing protocol that uses the Bellman-Ford algorithm and periodic updates, its usefulness is limited in many of today's networks.
Therefore, Cisco enhanced IGRP with a new algorithm, DUAL and other features. The commands for both IGRP and EIGRP are similar, and in many cases identical. This allows for easy migration from IGRP to EIGRP. Cisco discontinued IGRP starting with IOS 12.2(13)T and 12.2(R1s4)S.
Although discussed in more detail throughout this chapter, let us examine some of the differences between a traditional distance vector routing protocol such as RIP and IGRP, and the enhanced distance vector routing protocol, EIGRP.
The figure summarizes the main differences between a traditional distance vector routing protocol, such as RIP, and the enhanced distance vector routing protocol EIGRP.
The Algorithm
Traditional distance vector routing protocols all use some variant of the Bellman-Ford or Ford-Fulkerson algorithm. These protocols, such as RIP and IGRP, age out individual routing entries, and therefore need to periodically send routing table updates.
EIGRP uses the Diffusing Update Algorithm (DUAL). Although still a distance vector routing protocol, EIGRP with DUAL implements features not found in traditional distance vector routing protocols. EIGRP does not send periodic updates and route entries do not age out. Instead, EIGRP uses a lightweight Hello protocol to monitor connection status with its neighbors. Only changes in the routing information, such as a new link or a link becoming unavailable cause a routing update to occur. EIGRP routing updates are still vectors of distances transmitted to directly connected neighbors.
Path Determination
Traditional distance vector routing protocols such as RIP and IGRP keep track of only the preferred routes; the best path to a destination network. If the route becomes unavailable, the router waits for another routing update with a path to this remote network.
EIGRP's DUAL maintains a topology table separate from the routing table, which includes both the best path to a destination network and any backup paths that DUAL has determined to be loop-free. Loop-free means that the neighbor does not have a route to the destination network that passes through this router.
Later in this chapter, you will see that for a route to be considered as a valid loop-free backup path by DUAL, it must meet a requirement known as the feasibility condition. Any backup path that meets this condition is guaranteed to be loop-free. Because EIGRP is a distance vector routing protocol, it is possible that there might be loop-free backup paths to a destination network that do not meet the feasibility condition. These paths are therefore not included in the topology table as a valid loop-free backup path by DUAL.
If a route becomes unavailable, DUAL will search its topology table for a valid backup path. If one exists, that route is immediately entered into the routing table. If one does not exist, DUAL performs a network discovery process to see if there happens to be a backup path that did not meet the requirement of the feasibility condition. This process is discussed more thoroughly later in this chapter.
Convergence
Traditional distance vector routing protocols such as RIP and IGRP use periodic updates. Due to the unreliable nature of periodic updates, traditional distance vector routing protocols are prone to routing loops and the count-to-infinity problem. RIP and IGRP use several mechanisms to help avoid these problems including holddown timers, which cause long convergence times.
EIGRP does not use holddown timers. Instead, loop-free paths are achieved through a system of route calculations (diffusing computations) that are performed in a coordinated fashion among the routers. The detail of how this is done is beyond the scope of this course, but the result is faster convergence than traditional distance vector routing protocols.
9.1.1 - EIGRP: An Enhanced Distance Vector Routing Protocol
The diagram compares IGRP and EIGRP.
IGRP was developed by Cisco in 1985. Starting in 2005, it was no longer supported in Cisco I O S 12.2(13)T and 12.2(R1s4)S.
EIGRP was developed by Cisco in 1992 and was released with Cisco I O S 9.2.1.
Traditional distance vector routing protocols such as IGRP have the following characteristics:
- Use the Bellman-Ford or Ford-Fulkerson algorithm.
- Age out routing entries and use periodic updates.
- Keep track of only the best routes, and the best path to a destination network.
- When a route becomes unavailable, the router must wait for a new routing update.
- Slower convergence because of hold-down timers.
The enhanced distance vector routing protocol, EIGRP, has the following characteristics:
- Uses the Diffusing Update Algorithm (DUAL).
- Does not age out routing entries or use periodic updates.
- Maintains a topology table separate from the routing table, which includes the best path for any loop-free backup paths.
- When a route becomes unavailable, DUAL uses a backup path if one exists in the topology table.
- Faster convergence because of the absence of hold-down timers and a system of coordinated route calculations.
The diagram compares IGRP and EIGRP.
IGRP was developed by Cisco in 1985. Starting in 2005, it was no longer supported in Cisco I O S 12.2(13)T and 12.2(R1s4)S.
EIGRP was developed by Cisco in 1992 and was released with Cisco I O S 9.2.1.
Traditional distance vector routing protocols such as IGRP have the following characteristics:
- Use the Bellman-Ford or Ford-Fulkerson algorithm.
- Age out routing entries and use periodic updates.
- Keep track of only the best routes, and the best path to a destination network.
- When a route becomes unavailable, the router must wait for a new routing update.
- Slower convergence because of hold-down timers.
The enhanced distance vector routing protocol, EIGRP, has the following characteristics:
- Uses the Diffusing Update Algorithm (DUAL).
- Does not age out routing entries or use periodic updates.
- Maintains a topology table separate from the routing table, which includes the best path for any loop-free backup paths.
- When a route becomes unavailable, DUAL uses a backup path if one exists in the topology table.
- Faster convergence because of the absence of hold-down timers and a system of coordinated route calculations.
9.1.2 EIGRP Message Format
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Roll over the fields in the Encapsulated EIGRP Message to see the encapsulation process.
The data portion of an EIGRP message is encapsulated in a packet. This data field is called Type/Length/Value or TLV. As shown in the figure, the types of TLVs relevant to this course are EIGRP Parameters, IP Internal Routes, and IP External Routes. The components of the TLV data field are discussed further on the next page.
The EIGRP packet header is included with every EIGRP packet, regardless of its type. The EIGRP packet header and TLV are then encapsulated in an IP packet. In the IP packet header, the protocol field is set to 88 to indicate EIGRP, and the destination address is set to the multicast 224.0.0.10. If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: 01-00-5E-00-00-0A.
9.1.2 - EIGRP Message Format
The diagram depicts an encapsulated EIGRP message starting with the data link frame header encapsulating the IP packet header, which encapsulates the EIGRP packet header that contains the following type, length, value types.
Data Link Frame:
- MAC Destination Address = Multicast: 01-00-5E-00-00-0A
- MAC Source Address = Address of sending interface
IP Packet:
- IP Source Address = Address of sending interface
- IP Destination Address = Multicast: 224.0.0.10
- Protocol field = 88 for EIGRP
EIGRP Packet Header:
- Opcode for EIGRP packet type
- A S Number
Type, Length, Value Types:
Some types include:
- 0x0001 EIGRP Parameters
- 0x0102 IP Internal Routes
- 0x0103 IP External Routes
The diagram depicts an encapsulated EIGRP message starting with the data link frame header encapsulating the IP packet header, which encapsulates the EIGRP packet header that contains the following type, length, value types.
Data Link Frame:
- MAC Destination Address = Multicast: 01-00-5E-00-00-0A
- MAC Source Address = Address of sending interface
IP Packet:
- IP Source Address = Address of sending interface
- IP Destination Address = Multicast: 224.0.0.10
- Protocol field = 88 for EIGRP
EIGRP Packet Header:
- Opcode for EIGRP packet type
- A S Number
Type, Length, Value Types:
Some types include:
- 0x0001 EIGRP Parameters
- 0x0102 IP Internal Routes
- 0x0103 IP External Routes
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Note: In the following discussion of EIGRP messages, many fields are beyond the scope of this course. All fields are shown to provide an accurate picture of the EIGRP message format. However, only the fields relevant to the CCNA candidate are discussed.
Click EIGRP Packet Header in the figure.
Every EIGRP message includes the header. Important fields for our discussion include the Opcode field and the Autonomous System Number field. Opcode specifies the EIGRP packet type:
- Update
- Query
- Reply
- Hello
EIGRP packet types are discussed later in this chapter.
Click TLV: EIGRP Parameters in the figure.
The EIGRP parameters message includes the weights that EIGRP uses for its composite metric. By default, only bandwidth and delay are weighted. Both are equally weighted, therefore, the K1 field for bandwidth and the K3 field for delay are both set to 1. The other K values are set to zero. Metric calculations are further discussed later in this chapter.
The Hold Time is the amount of time the EIGRP neighbor receiving this message should wait before considering the advertising router to be down. Hold Time is discussed in more detail later in this chapter.
Click TLV: IP Internal in the figure.
The IP Internal message is used to advertise EIGRP routes within an autonomous system. Important fields for our discussion include: the metric fields (Delay and Bandwidth), the subnet mask field (Prefix Length), and the Destination field.
Delay is calculated as the sum of delays from source to destination in units of 10 microseconds. Bandwidth is the lowest configured bandwidth of any interface along the route.
The subnet mask is specified as the prefix length or the number of network bits in the subnet mask. For example, the prefix length for the subnet mask 255.255.255.0 is 24 because 24 is the number of network bits.
The Destination field stores the address of the destination network. Although only 24 bits are shown in this figure, this field varies based on the value of the network portion of the 32-bit network address. For example, the network portion of 10.1.0.0/16 is 10.1. Therefore, the Destination field stores the first 16 bits. Because the minimum length of this field is 24 bits, the remainder of the field is padded with zeros. If a network address is longer than 24 bits (192.168.1.32/27, for example), then the Destination field is extended for another 32 bits (for a total of 56 bits) and the unused bits are padded with zeros.
Click TLV: IP External in the figure.
The IP External message is used when external routes are imported into the EIGRP routing process. In this chapter, we will import or redistribute a default static route into EIGRP. Notice that the bottom half of the IP External TLV includes all the fields used by the IP Internal TLV.
Note: Some EIGRP literature may incorrectly state that the Maximum Transmission Unit (MTU) is one of the metrics used by EIGRP. MTU is not a metric used by EIGRP. The MTU is included in the routing updates but it is not used to determine the routing metric.
9.1.2 - EIGRP Message Format
The diagram depicts key components of the encapsulated EIGRP message.
EIGRP packet header key components:
- Opcode: EIGRP Packet Type: Update (1), Query (3), Reply (4), Hello (5)
- Autonomous System Number: ID for this EIGRP routing process
TLV: EIGRP Parameters key components:
- K1 and K3: Weights for bandwidth and delay; set to 1
- Hold Time: Maximum time router should wait for the next hello
TLV: IP Internal key components:
- Delay: Sum of delays in units of 10 microseconds from source to destination; 0xFFFFFFFF indicates unreachable route
- Bandwidth: Lowest configured bandwidth of any interface along the route
- Prefix Length: Specifies the number of network bits in the subnet mask
- Destination: The destination address of the route
TLV: IP External key components:
Value fields used to track the external source of a route include:
- Next hop
- Originating routers
- Originating Autonomous System number
- Arbitrary tag
- External protocol metric
- Reserved
- External Protocol ID
- Flags
In addition, the same value fields used in the IP Internal TLV are also present.
The diagram depicts key components of the encapsulated EIGRP message.
EIGRP packet header key components:
- Opcode: EIGRP Packet Type: Update (1), Query (3), Reply (4), Hello (5)
- Autonomous System Number: ID for this EIGRP routing process
TLV: EIGRP Parameters key components:
- K1 and K3: Weights for bandwidth and delay; set to 1
- Hold Time: Maximum time router should wait for the next hello
TLV: IP Internal key components:
- Delay: Sum of delays in units of 10 microseconds from source to destination; 0xFFFFFFFF indicates unreachable route
- Bandwidth: Lowest configured bandwidth of any interface along the route
- Prefix Length: Specifies the number of network bits in the subnet mask
- Destination: The destination address of the route
TLV: IP External key components:
Value fields used to track the external source of a route include:
- Next hop
- Originating routers
- Originating Autonomous System number
- Arbitrary tag
- External protocol metric
- Reserved
- External Protocol ID
- Flags
In addition, the same value fields used in the IP Internal TLV are also present.
9.1.3 Protocol Dependent Modules (PDM)
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EIGRP has the capability for routing several different protocols including IP, IPX, and AppleTalk using protocol-dependent modules (PDM). PDMs are responsible for the specific routing tasks for each Network layer protocol.
For example:
- The IP-EIGRP module is responsible for sending and receiving EIGRP packets that are encapsulated in IP and for using DUAL to build and maintain the IP routing table. As you can see in the figure, EIGRP uses different EIGRP packets and maintains separate neighbor, topology, and routing tables for each Network layer protocol.
- The IPX EIGRP module is responsible for exchanging routing information about IPX networks with other IPX EIGRP routers. IPX EIGRP and Appletalk EIGRP are not included in this course.
9.1.3 - Protocol Dependent Modules (PDM)
The diagram depicts key components of the encapsulated EIGRP message.
EIGRP packet header key components:
- Opcode: EIGRP Packet Type: Update (1), Query (3), Reply (4), Hello (5)
- Autonomous System Number: ID for this EIGRP routing process
TLV: EIGRP Parameters key components:
- K1 and K3: Weights for bandwidth and delay; set to 1
- Hold Time: Maximum time router should wait for the next hello
TLV: IP Internal key components:
- Delay: Sum of delays in units of 10 microseconds from source to destination; 0xFFFFFFFF indicates unreachable route
- Bandwidth: Lowest configured bandwidth of any interface along the route
- Prefix Length: Specifies the number of network bits in the subnet mask
- Destination: The destination address of the route
TLV: IP External key components:
Value fields used to track the external source of a route include:
- Next hop
- Originating routers
- Originating Autonomous System number
- Arbitrary tag
- External protocol metric
- Reserved
- External Protocol ID
- Flags
In addition, the same value fields used in the IP Internal TLV are also present.
The diagram depicts key components of the encapsulated EIGRP message.
EIGRP packet header key components:
- Opcode: EIGRP Packet Type: Update (1), Query (3), Reply (4), Hello (5)
- Autonomous System Number: ID for this EIGRP routing process
TLV: EIGRP Parameters key components:
- K1 and K3: Weights for bandwidth and delay; set to 1
- Hold Time: Maximum time router should wait for the next hello
TLV: IP Internal key components:
- Delay: Sum of delays in units of 10 microseconds from source to destination; 0xFFFFFFFF indicates unreachable route
- Bandwidth: Lowest configured bandwidth of any interface along the route
- Prefix Length: Specifies the number of network bits in the subnet mask
- Destination: The destination address of the route
TLV: IP External key components:
Value fields used to track the external source of a route include:
- Next hop
- Originating routers
- Originating Autonomous System number
- Arbitrary tag
- External protocol metric
- Reserved
- External Protocol ID
- Flags
In addition, the same value fields used in the IP Internal TLV are also present.
9.1.4 RTP and EIGRP Packet Types
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Reliable Transport Protocol (RTP) is the protocol used by EIGRP for the delivery and reception of EIGRP packets. EIGRP was designed as a Network layer independent routing protocol; therefore, it cannot use the services of UDP or TCP because IPX and Appletalk do not use protocols from the TCP/IP protocol suite. The figure shows conceptually how RTP operates.
Although "Reliable" is part of its name, RTP includes both reliable delivery and unreliable delivery of EIGRP packets, similar to TCP and UDP, respectively. Reliable RTP requires an acknowledgement to be returned by the receiver to the sender. An unreliable RTP packet does not require an acknowledgement.
RTP can send packets either as a unicast or a multicast. Multicast EIGRP packets use the reserved multicast address of 224.0.0.10.
9.1.4 - RTP and EIGRP Packet Types
The diagram depicts how EIGRP replaces TCP with Reliable Transport Protocol (RTP) for the delivery and reception of EIGRP packets. PDMs, DUAL, neighbor discovery and recovery, and RTP are contained in the Network Layer encapsulation as either IP, IPX, or AppleTalk.
The diagram depicts how EIGRP replaces TCP with Reliable Transport Protocol (RTP) for the delivery and reception of EIGRP packets. PDMs, DUAL, neighbor discovery and recovery, and RTP are contained in the Network Layer encapsulation as either IP, IPX, or AppleTalk.
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EIGRP Packet Types
EIGRP uses five different packet types, some in pairs.
Click Hello in the figure.
Hello packets are used by EIGRP to discover neighbors and to form adjacencies with those neighbors. EIGRP hello packets are multicasts and use unreliable delivery. EIGRP Hello packets are discussed in a later section.
Click Update and ACK in the figure.
Update packets are used by EIGRP to propagate routing information. Unlike RIP, EIGRP does not send periodic updates. Update packets are sent only when necessary. EIGRP updates contain only the routing information needed and are sent only to those routers that require it. EIGRP update packets use reliable delivery. Update packets are sent as a multicast when required by multiple routers, or as a unicast when required by only a single router. In the figure, because the links are point-to-point, the updates are sent as unicasts.
Acknowledgement (ACK) packets are sent by EIGRP when reliable delivery is used. RTP uses reliable delivery for EIGRP update, query, and reply packets. EIGRP acknowledgement packets contain a nonzero acknowledgment number and always are sent by using a unicast address.
In the figure, R2 has lost connectivity to the LAN attached to its FastEthernet interface. R2 immediately sends an Update to R1 and R3 noting the downed route. R1 and R3 respond with an acknowledgement.
Click Query and Reply in the figure.
Query and reply packets are used by DUAL when searching for networks and other tasks. Queries and replies use reliable delivery. Queries use multicast, whereas replies are always sent as unicast. DUAL is discussed in a later section. Query and reply packets are discussed in more detail in CCNP.
In the figure, R2 has lost connectivity to the LAN and it sends out queries to all EIGRP neighbors searching for any possible routes to the LAN. Because queries use reliable delivery, the receiving router must return an EIGRP acknowledgement. (To keep this example simple, acknowledgements were omitted in the graphic.)
All neighbors must send a reply regardless of whether or not they have a route to the downed network. Because replies also use reliable delivery, routers such as R2, must send an acknowledgement.
Note: You may be wondering why R2 would send out a query for a network it knows is down. Actually, only the interface attached to the network is down. Another router could be attached to the same LAN. Therefore, R2 queries for such a router before completely removing the network from its database.
9.1.4 - RTP and EIGRP Packet Types
The diagram depicts EIGRP packet types. The diagram is based on the following network topology.
Network Topology:
There are three routers, R1, R2, and R3. Each router has a LAN attached. R1 is connected to R2 via a WAN link. R1 is connected to R3 via a WAN link. R2 is connected to R3 via a WAN link.
Five types of EIGRP packets are shown passing between the routers: Hello, Update, ACK, Query, and Reply. Key points are listed for each packet type.
Hello packet:
- Used to discover neighbors and form adjacencies
- Unreliable, so no response is required from recipient
Update packet:
- Used to propagate routing information after a change Acknowledgement (ACK) packet
- Automatically sent back when reliable RTP is used
Query packet:
- Used by DUAL when searching for networks or other tasks
Reply packet:
- Automatically sent in response to a Query packet
Acknowledgement (ACK) packet:
- Automatically sent back when reliable RTP is used
The diagram depicts EIGRP packet types. The diagram is based on the following network topology.
Network Topology:
There are three routers, R1, R2, and R3. Each router has a LAN attached. R1 is connected to R2 via a WAN link. R1 is connected to R3 via a WAN link. R2 is connected to R3 via a WAN link.
Five types of EIGRP packets are shown passing between the routers: Hello, Update, ACK, Query, and Reply. Key points are listed for each packet type.
Hello packet:
- Used to discover neighbors and form adjacencies
- Unreliable, so no response is required from recipient
Update packet:
- Used to propagate routing information after a change Acknowledgement (ACK) packet
- Automatically sent back when reliable RTP is used
Query packet:
- Used by DUAL when searching for networks or other tasks
Reply packet:
- Automatically sent in response to a Query packet
Acknowledgement (ACK) packet:
- Automatically sent back when reliable RTP is used
9.1.5 Hello Protocol
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Before any EIGRP packets can be exchanged between routers, EIGRP must first discover its neighbors. EIGRP neighbors are other routers running EIGRP on shared, directly connected networks.
EIGRP routers discover neighbors and establish adjacencies with neighbor routers using the Hello packet. On most networks EIGRP Hello packets are sent every 5 seconds. On multipoint nonbroadcast multiaccess networks (NBMA) such as X.25, Frame Relay, and ATM interfaces with access links of T1 (1.544 Mbps) or slower, Hellos are unicast every 60 seconds. An EIGRP router assumes that as long as it is receiving Hello packets from a neighbor, the neighbor and its routes remain viable.
Holdtime tells the router the maximum time the router should wait to receive the next Hello before declaring that neighbor as unreachable. By default, the hold time is three times the Hello interval, or 15 seconds on most networks and 180 seconds on low speed NBMA networks. If the hold time expires, EIGRP will declare the route as down and DUAL will search for a new path by sending out queries.
9.1.5 - Hello Protocol
The diagram depicts EIGRP default Hello intervals and hold times for various link types.
Bandwidth: 1.544 Mbps
Example Link: Multipoint Frame Relay
Default Hello Interval: 60 seconds
Default Hold Time: 180 seconds
Bandwidth: Greater than 1.544 Mbps
Example Link: T1, Ethernet
Default Hello Interval: 5 seconds
Default Hold Time: 15 seconds
The diagram depicts EIGRP default Hello intervals and hold times for various link types.
Bandwidth: 1.544 Mbps
Example Link: Multipoint Frame Relay
Default Hello Interval: 60 seconds
Default Hold Time: 180 seconds
Bandwidth: Greater than 1.544 Mbps
Example Link: T1, Ethernet
Default Hello Interval: 5 seconds
Default Hold Time: 15 seconds
9.1.6 EIGRP Bounded Updates
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EIGRP uses the term partial or bounded when referring to its update packets. Unlike RIP, EIGRP does not send periodic updates. Instead, EIGRP sends its updates only when the metric for a route changes.
The term partial means that the update only includes information about the route changes. EIGRP sends these incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table.
The term bounded refers to the propagation of partial updates sent only to those routers that are affected by the change. The partial update is automatically "bounded" so that only those routers that need the information are updated.
By sending only the routing information that is needed and only to those routers that need it, EIGRP minimizes the bandwidth required to send EIGRP packets.
9.1.6 - EIGRP Bounded Updates
The diagram depicts characteristics of EIGRP updates. EIGRP updates are partial and bounded. Updates are partial because they only include information about route changes. They are bounded because only those routers affected by the change receive the update.
The diagram depicts characteristics of EIGRP updates. EIGRP updates are partial and bounded. Updates are partial because they only include information about route changes. They are bounded because only those routers affected by the change receive the update.
9.1.7 DUAL: An Introduction
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Diffusing Update Algorithm (DUAL) is the convergence algorithm used by EIGRP instead of the Bellman-Ford or Ford Fulkerson algorithms used by other distance vector routing protocols, like RIP. DUAL is based on research conducted at SRI International, using calculations that were first proposed by E.W. Dijkstra and C.S. Scholten. The most prominent work with DUAL has been done by J.J. Garcia-Luna-Aceves.
Routing loops, even temporary ones, can be extremely detrimental to network performance. Distance vector routing protocols such as RIP prevent routing loops with hold-down timers and split horizon. Although EIGRP uses both of these techniques, it uses them somewhat differently; the primary way that EIGRP prevents routing loops is with the DUAL algorithm.
Click Play to view the basic operation of DUAL.
The DUAL algorithm is used to obtain loop-freedom at every instant throughout a route computation. This allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by the topology changes are not involved in the recomputation. This method provides EIGRP with faster convergence times than other distance vector routing protocols.
The decision process for all route computations is done by the DUAL Finite State Machine. In general terms, a finite state machine (FSM) is a model of behavior composed of a finite number of states, transitions between those states, and events or actions that create the transitions.
The DUAL FSM tracks all routes, uses its metric to select efficient, loop-free paths, and selects the routes with the least cost path to insert into the routing table. The DUAL FSM will be discussed in more detail later in this chapter.
Because recomputation of the DUAL algorithm can be processor-intensive, it is advantageous to avoid recomputation whenever possible. Therefore, DUAL maintains a list of backup routes it has already determined to be loop-free. If the primary route in the routing table fails, the best backup route is immediately added to the routing table.
9.1.7 - DUAL: An Introduction
The animation depicts the operation of Diffusing Update Algorithm (DUAL). The animation is based on the network topology described in 9.1.4 diagram 2.
As the animation progresses:
1. The LAN on router R2 goes down.
2. R2 sends a partial update to routers R1 and R3.
3. Routers R1 and R3 send ACK packets back to R2.
4. R2 sends a Query to routers R1 and R3.
5. Routers R1 and R3 send ACK packets and replies back to R2.
6. R2 sends ACK's to routers R1 and R3.
7. DUAL runs on each router to update the routing table.
The animation depicts the operation of Diffusing Update Algorithm (DUAL). The animation is based on the network topology described in 9.1.4 diagram 2.
As the animation progresses:
1. The LAN on router R2 goes down.
2. R2 sends a partial update to routers R1 and R3.
3. Routers R1 and R3 send ACK packets back to R2.
4. R2 sends a Query to routers R1 and R3.
5. Routers R1 and R3 send ACK packets and replies back to R2.
6. R2 sends ACK's to routers R1 and R3.
7. DUAL runs on each router to update the routing table.
9.1.8 Administrative Distance
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As you know from Chapter 3, "Introduction to Dynamic Routing Protocols," administrative distance (AD) is the trustworthiness (or preference) of the route source. EIGRP has a default administrative distance of 90 for internal routes and 170 for routes imported from an external source, such as default routes. When compared to other interior gateway protocols (IGPs), EIGRP is the most preferred by the Cisco IOS because it has the lowest administrative distance.
Notice in the figure that EIGRP has a third AD value, of 5, for summary routes. Later in this chapter, you will learn how to configure EIGRP summary routes.
9.1.8 - Administrative Distance
The diagram depicts the default administrative distance (A D) for various route sources and protocols.
Route Source: Connected
Administrative Distance: 0
Route Source: Static
Administrative Distance: 1
Route Source: EIGRP summary route
Administrative Distance: 5
Route Source: External BGP
Administrative Distance: 20
Route Source: Internal EIGRP
Administrative Distance: 90
Route Source: IGRP
Administrative Distance: 100
Route Source: OSPF
Administrative Distance: 110
Route Source: I S-I S
Administrative Distance: 115
Route Source: RIP
Administrative Distance: 120
Route Source: External EIGRP
Administrative Distance: 170
Route Source: Internal BGP
Administrative Distance: 200
The diagram depicts the default administrative distance (A D) for various route sources and protocols.
Route Source: Connected
Administrative Distance: 0
Route Source: Static
Administrative Distance: 1
Route Source: EIGRP summary route
Administrative Distance: 5
Route Source: External BGP
Administrative Distance: 20
Route Source: Internal EIGRP
Administrative Distance: 90
Route Source: IGRP
Administrative Distance: 100
Route Source: OSPF
Administrative Distance: 110
Route Source: I S-I S
Administrative Distance: 115
Route Source: RIP
Administrative Distance: 120
Route Source: External EIGRP
Administrative Distance: 170
Route Source: Internal BGP
Administrative Distance: 200
9.1.9 Authentication
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Like other routing protocols, EIGRP can be configured for authentication. RIPv2, EIGRP, OSPF, IS-IS, and BGP can all be configured to encrypt and authenticate their routing information.
It is good practice to authenticate transmitted routing information. This practice ensures that routers will only accept routing information from other routers that have been configured with the same password or authentication information.
Note: Authentication does not encrypt the router's routing table.
As stated in previous chapters, configuring routing protocols to use authentication will be discussed in a later course.
9.1.9 - Authentication
The diagram depicts EIGRP's encryption and authentication of routing information. The diagram is based on the network topology described in 9.1.4 diagram 2, except that the routers have locks on them denoting the encryption and authentication of EIGRP packets between routers.
The diagram depicts EIGRP's encryption and authentication of routing information. The diagram is based on the network topology described in 9.1.4 diagram 2, except that the routers have locks on them denoting the encryption and authentication of EIGRP packets between routers.
9.2 Basic EIGRP Configuration
9.2.1 EIGRP Network TopologyPage 1:
The figure shows our topology from previous chapters, but now includes the addition of the ISP router. Notice that both the R1 and R2 routers have subnets that are part of the 172.16.0.0/16 classful network, a class B address. The fact that 172.16.0.0 is a class B address is only relevant because EIGRP automatically summarizes at classful boundaries, similar to RIP.
Click R1, R2, and R3 to see each router's starting configuration.
Notice that the ISP router does not physically exist in our configurations. The connection between R2 and ISP is represented with a loopback interface on router R2. Remember from Chapter 7, "RIPv2," that a loopback interface can be used to represent an interface on a router that does not have any actual connection to a physical link on the network. Loopback addresses can be verified with the ping command and included in routing updates.
Note: Loopback interfaces also have specific uses with some routing protocols, as we will see in Chapter 11, OSPF.
9.2.1 - EIGRP Network Topology
The diagram depicts the EIGRP topology used with this chapter. An addressing table and the commands used to configure interfaces are shown.
The diagram is based on the following network topology.
Network Topology:
There are three routers, R1, R2, and R3, interconnected by WAN links. Each router has a LAN attached. An ISP router is connected to R2, but is actually simulated using a loopback interface on R2.
R1 FA0/0 is connected to LAN 172.16.1.0/24.
R1 S0/0/0 (DCE) is connected to R2 S0/0/0 via a 64 kilobit per second WAN link.
R1 S0/0/1 is connected to R3 S0/0/0 via a 1,544 kilobit per second WAN link.
R2 S0/0/1 (DCE) is connected to R3 S0/0/1 via a 1,024 kilobit per second WAN link.
Addressing table:
Router R1 interface addresses:
Interface FA0/0 IP Address: 172.16.1.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 172.16.3.1, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.5, subnet mask 255.255.255.252
Router R2 interface addresses:
Interface FA0/0 IP Address: 172.16.2.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 172.16.3.2, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.9, subnet mask 255.255.255.252
Interface L o 1 IP Address: 10.1.1.1, subnet mask 255.255.255.252
Router R3 interface addresses:
Interface FA0/0 IP Address: 192.168.1.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 192.168.10.6, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.10, subnet mask 255.255.255.252
The diagram depicts the EIGRP topology used with this chapter. An addressing table and the commands used to configure interfaces are shown.
The diagram is based on the following network topology.
Network Topology:
There are three routers, R1, R2, and R3, interconnected by WAN links. Each router has a LAN attached. An ISP router is connected to R2, but is actually simulated using a loopback interface on R2.
R1 FA0/0 is connected to LAN 172.16.1.0/24.
R1 S0/0/0 (DCE) is connected to R2 S0/0/0 via a 64 kilobit per second WAN link.
R1 S0/0/1 is connected to R3 S0/0/0 via a 1,544 kilobit per second WAN link.
R2 S0/0/1 (DCE) is connected to R3 S0/0/1 via a 1,024 kilobit per second WAN link.
Addressing table:
Router R1 interface addresses:
Interface FA0/0 IP Address: 172.16.1.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 172.16.3.1, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.5, subnet mask 255.255.255.252
Router R2 interface addresses:
Interface FA0/0 IP Address: 172.16.2.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 172.16.3.2, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.9, subnet mask 255.255.255.252
Interface L o 1 IP Address: 10.1.1.1, subnet mask 255.255.255.252
Router R3 interface addresses:
Interface FA0/0 IP Address: 192.168.1.1, subnet mask 255.255.255.0
Interface S0/0/0 IP Address: 192.168.10.6, subnet mask 255.255.255.252
Interface S0/0/1 IP Address: 192.168.10.10, subnet mask 255.255.255.252
9.2.2 Autonomous Systems and Process IDs
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Autonomous System
An autonomous system (AS) is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 "presents a common routing policy" for all of these companies when advertising routes to ISP2.
The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930. AS numbers are assigned by the Internet Assigned Numbers Authority (IANA), the same authority that assigns IP address space. You learned about IANA and its Regional Internet Registries (RIRs) in a previous course. The local RIR is responsible for assigning an AS number to an entity from its block of assigned AS numbers. Prior to 2007, AS numbers were 16-bit numbers, ranging from 0 to 65535. Now 32-bit AS numbers are assigned, increasing the number of available AS numbers to over 4 billion.
Who needs an autonomous system number? Usually ISPs (Internet Service Providers), Internet backbone providers, and large institutions connecting to other entities that also have an AS number. These ISPs and large institutions use the exterior gateway routing protocol Border Gateway Protocol, or BGP, to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.
The vast majority of companies and institutions with IP networks do not need an AS number because they come under the control of a larger entity such as an ISP. These companies use interior gateway protocols such as RIP, EIGRP, OSPF, and IS-IS to route packets within their own networks. They are one of many independent and separate networks within the autonomous system of the ISP. The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems.
9.2.2 - Autonomous Systems and Process ID's
The diagram depicts the ISP1 Autonomous System (A S) connected to the ISP2 A S.
The diagram is based on the following network topology.
Network Topology:
A circle labeled ISP1 A S 64515 contains four companies, each with its own cloud and four interconnected routers. Company A and Company C run the EIGRP routing protocol and are connected to ISP1 through router R1. Company B runs OSPF and is connected to ISP1 through router R2. Company D runs EIGRP and is connected to ISP1 through router R2. ISP1 router R1 is connected to ISP1 router R2. ISP1 router R2 is connected to ISP1 router R3 at the edge of the A S.
A circle labeled ISP2 A S 64239 has one router at the edge. This router is connected to ISP1 router R3. The BGP routing protocol is used between these two edge routers.
The diagram depicts the ISP1 Autonomous System (A S) connected to the ISP2 A S.
The diagram is based on the following network topology.
Network Topology:
A circle labeled ISP1 A S 64515 contains four companies, each with its own cloud and four interconnected routers. Company A and Company C run the EIGRP routing protocol and are connected to ISP1 through router R1. Company B runs OSPF and is connected to ISP1 through router R2. Company D runs EIGRP and is connected to ISP1 through router R2. ISP1 router R1 is connected to ISP1 router R2. ISP1 router R2 is connected to ISP1 router R3 at the edge of the A S.
A circle labeled ISP2 A S 64239 has one router at the edge. This router is connected to ISP1 router R3. The BGP routing protocol is used between these two edge routers.
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Process ID
Both EIGRP and OSPF use a process ID to represent an instance of their respective routing protocol running on the router.
Router(config)#router eigrp autonomous-system
Although EIGRP refers to the parameter as an "autonomous-system" number, it actually functions as a process ID. This number is not associated with an autonomous system number discussed previously and can be assigned any 16-bit value.
Router(config)#router eigrp 1
In this example, the number 1 identifies this particular EIGRP process running on this router. In order to establish neighbor adjacencies, EIGRP requires all routers in the same routing domain to be configured with the same process ID. Typically, only a single process ID of any routing protocol would be configured on a router.
Note: RIP does not use process IDs; therefore, it can only support a single instance of RIP. Both EIGRP and OSPF can support multiple instances of each routing protocol, although this type of multiple routing protocol implementation is not usually needed or recommended.
9.2.2 - Autonomous Systems and Process ID's
The diagram depicts an EIGRP topology that uses a single process ID. The diagram is based on the following network topology.
Network Topology:
Four routers are interconnected to four other routers in a cloud that is labeled EIGRP 1.
The following commands are used:
R1(config)#router e i g r p ?
1-65535 Autonomous system number
R1(config)#router e i g r p 1
Although the Cisco I O S refers to the router e i g r p parameter as an "Autonomous system number", this parameter configures an EIGRP process, an instance of EIGRP running on the router, and has nothing to do with A S configuration in ISP routers.
The diagram depicts an EIGRP topology that uses a single process ID. The diagram is based on the following network topology.
Network Topology:
Four routers are interconnected to four other routers in a cloud that is labeled EIGRP 1.
The following commands are used:
R1(config)#router e i g r p ?
1-65535 Autonomous system number
R1(config)#router e i g r p 1
Although the Cisco I O S refers to the router e i g r p parameter as an "Autonomous system number", this parameter configures an EIGRP process, an instance of EIGRP running on the router, and has nothing to do with A S configuration in ISP routers.
9.2.3 The router eigrp command
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The router eigrp autonomous-system global configuration command enables EIGRP. The autonomous system parameter is a number chosen by the network administrator between 1 and 65535. The number chosen is the process ID number and is important because all routers in this EIGRP routing domain must use the same process ID number (autonomous-system number).
Click Router Output in the figure.
As you can see from the topology and router output in the figure, we will enable EIGRP on all three routers using the process ID of 1.
9.2.3 - The router e i g r p command
The diagram depicts the basic EIGRP topology for this chapter and the configuration of the EIGRP 1 routing process on each router. The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts the basic EIGRP topology for this chapter and the configuration of the EIGRP 1 routing process on each router. The diagram is based on the network topology described in 9.2.1 diagram 1.
9.2.4 The network Command
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The network command in EIGRP has the same function as in other IGP routing protocols:
- Any interface on this router that matches the network address in the network command will be enabled to send and receive EIGRP updates.
- This network (or subnet) will be included in EIGRP routing updates.
The network command is used in router configuration mode.
Router(config-router)#network network-address
The network-address is the classful network address for this interface. The figure shows the network commands configured for R1 and R2. R3 will be configured on the next page. In the figure, a single classful network statement is used on R1 to include both 172.16.1.0/24 and 172.16.3.0/30 subnets:
R1(config-router)#network 172.16.0.0
When EIGRP is configured on R2, DUAL sends a notification message to the console stating that a neighbor relationship with another EIGRP router has been established. This new adjacency happens automatically because both R1 and R2 are using the same eigrp 1 routing process and both routers are now sending updates on the 172.16.0.0 network.
R2(config-router)#network 172.16.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 172.16.3.1 (Serial0/0) is up: new adjacency
9.2.4 - The network Command
The diagram depicts using the network command to configure EIGRP networks. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are shown for R1:
R1(config)#router e i g r p 1
R1(config-router)#network 172.16.0.0
R1(config-router)#network 192.168.10.0
The following commands are shown for R2:
R2(config)#router e i g r p 1
R2(config-router)#network 172.16.0.0
An EIGRP message is displayed when the two routers begin to communicate.
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 172.16.3.1 (Serial0/0/0) is up: new adjacency
The diagram depicts using the network command to configure EIGRP networks. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are shown for R1:
R1(config)#router e i g r p 1
R1(config-router)#network 172.16.0.0
R1(config-router)#network 192.168.10.0
The following commands are shown for R2:
R2(config)#router e i g r p 1
R2(config-router)#network 172.16.0.0
An EIGRP message is displayed when the two routers begin to communicate.
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 172.16.3.1 (Serial0/0/0) is up: new adjacency
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The network Command with a Wildcard Mask
By default, when using the network command and a classful network address such as 172.16.0.0, all interfaces on the router that belong to that classful network address will be enabled for EIGRP. However, there may be times when the network administrator does not want to include all interfaces within a network when enabling EIGRP. To configure EIGRP to advertise specific subnets only, use the wildcard-mask option with the network command:
Router(config-router)#network network-address [wildcard-mask]
Think of a wildcard mask as the inverse of a subnet mask. The inverse of subnet mask 255.255.255.252 is 0.0.0.3. To calculate the inverse of the subnet mask, subtract the subnet mask from 255.255.255.255:
255.255.255.255
- 255.255.255.252
Subtract the subnet mask
---------------
0. 0. 0. 3
Wildcard mask
Click Router Output in the figure.
In the figure, R2 is configured with the subnet 192.168.10.8 and the wildcard mask 0.0.0.3.
R2(config-router)#network 192.168.10.8 0.0.0.3
Some IOS versions will also let you simply enter the subnet mask. For example, you might enter the following:
R2(config-router)#network 192.168.10.8 255.255.255.252
However, the IOS will then convert the command to the wildcard mask format, as can be verified with the show run command:
R2#show run
!
router eigrp 1
network 172.16.0.0
network 192.168.10.8 0.0.0.3
auto-summary
!
The figure also shows the configuration for R3. As soon as the classful network 192.168.10.0 is configured, R3 establishes adjacencies with both R1 and R2.
9.2.4 - The network Command
The diagram depicts configuring all EIGRP networks and subnetworks on R1, R2, and R3 using EIGRP process 1 and the network command. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are shown for R1:
R1(config)#router e i g r p 1
R1(config-router)#network 172.16.0.0
R1(config-router)#network 192.168.10.0
The following commands and messages are shown for R2:
R2(config)#router e i g r p 1
R2(config-router)#network 172.16.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 172.16.3.1 (Serial0/0/0) is up: new adjacency
R2(config-router)#network 192.168.10.8 0.0.0.3
The following commands and messages are shown for R3:
R3(config)#router e i g r p 1
R3(config-router)#network 192.168.10.0
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 192.168.10.5 (Serial0/0/0) is up: new adjacency
R3(config-router)#
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 192.168.10.9 (Serial0/0/1) is up: new adjacency
R3(config-router)#network 192.168.1.0
The diagram depicts configuring all EIGRP networks and subnetworks on R1, R2, and R3 using EIGRP process 1 and the network command. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are shown for R1:
R1(config)#router e i g r p 1
R1(config-router)#network 172.16.0.0
R1(config-router)#network 192.168.10.0
The following commands and messages are shown for R2:
R2(config)#router e i g r p 1
R2(config-router)#network 172.16.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 172.16.3.1 (Serial0/0/0) is up: new adjacency
R2(config-router)#network 192.168.10.8 0.0.0.3
The following commands and messages are shown for R3:
R3(config)#router e i g r p 1
R3(config-router)#network 192.168.10.0
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 192.168.10.5 (Serial0/0/0) is up: new adjacency
R3(config-router)#
%DUAL-5-NBRCHANGE: IP-EIGRP 1: Neighbor 192.168.10.9 (Serial0/0/1) is up: new adjacency
R3(config-router)#network 192.168.1.0
9.2.5 Verifying EIGRP
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Before any updates can be sent or received by EIGRP, routers must establish adjacencies with their neighbors. EIGRP routers establish adjacencies with neighbor routers by exchanging EIGRP Hello packets.
Use the show ip eigrp neighbors command to view the neighbor table and verify that EIGRP has established an adjacency with its neighbors. For each router, you should be able to see the IP address of the adjacent router and the interface that this router uses to reach that EIGRP neighbor. In the figure, we can verify that all routers have established the necessary adjacencies. Each router has two neighbors listed in the neighbor table.
The output from the show ip eigrp neighbor command includes:
- H column - Lists the neighbors in the order they were learned.
- Address - The IP address of the neighbor.
- Interface - The local interface on which this Hello packet was received.
- Hold - The current hold time. Whenever a Hello packet is received, this value is reset to the maximum hold time for that interface and then counts down to zero. If zero is reached, the neighbor is considered "down".
- Uptime - Amount of time since this neighbor was added to the neighbor table.
- SRTT (Smooth Round Trip Timer) and RTO (Retransmit Interval) - Used by RTP to manage reliable EIGRP packets. SRTT and RTO are discussed further in CCNP courses.
- Queue Count - Should always be zero. If more than zero, then EIGRP packets are waiting to be sent. Queue count is discussed further in CCNP courses.
- Sequence Number - Used to track updates, queries, and reply packets. Sequence numbers are discussed further in CCNP courses.
- Are both routers configured with the same EIGRP process ID?
- Is the directly connected network included in the EIGRP network statements?
- Is the passive-interface command configured to prevent EIGRP Hello packets on the interface?
9.2.5 - Verifying EIGRP
The diagram depicts the use of the show i p e i g r p neighbors command to display the neighbor table and verify that EIGRP has established an adjacency with its neighbors.
R2#show i p e i g r p neighbors
IP-EIGRP neighbors for process 1
H: 1
Address: 192.168.10.10
Interface: Serial0/0/1
Hold (sec): 10
Uptime: 00:01:41
SRTT (milliseconds): 20
RTO: 200
Q Count: 0
Sequence Number: 7
Type:
H: 0
Address: 172.16.3.1
Interface: Serial0/0/0
Hold (sec): 10
Uptime: 00:09:49
SRTT (milliseconds): 25
RTO: 200
Q Count: 0
Sequence Number: 28
Type:
In the output above:
- The address column is the address of neighbors.
- The interface column is the interface connected to a neighbor.
- The hold (sec) column is the amount of time left before the neighbor is considered down.
- The uptime column is the amount of time since adjacency was established.
The diagram depicts the use of the show i p e i g r p neighbors command to display the neighbor table and verify that EIGRP has established an adjacency with its neighbors.
R2#show i p e i g r p neighbors
IP-EIGRP neighbors for process 1
H: 1
Address: 192.168.10.10
Interface: Serial0/0/1
Hold (sec): 10
Uptime: 00:01:41
SRTT (milliseconds): 20
RTO: 200
Q Count: 0
Sequence Number: 7
Type:
H: 0
Address: 172.16.3.1
Interface: Serial0/0/0
Hold (sec): 10
Uptime: 00:09:49
SRTT (milliseconds): 25
RTO: 200
Q Count: 0
Sequence Number: 28
Type:
In the output above:
- The address column is the address of neighbors.
- The interface column is the interface connected to a neighbor.
- The hold (sec) column is the amount of time left before the neighbor is considered down.
- The uptime column is the amount of time since adjacency was established.
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As with RIP, the show ip protocols command can be used to verify that EIGRP is enabled. The show ip protocols command displays different types of output specific to each routing protocol. We will examine some of these details in later sections.
Click Router Output in the figure.
Notice that the output specifies the process ID used by EIGRP:
Routing Protocol is "eigrp 1"
Remember, the process ID must be the same on all routers for EIGRP to establish neighbor adjacencies and share routing information.
EIGRP's internal and external administrative distances are also displayed:
Distance: internal 90 external 170
9.2.5 - Verifying EIGRP
The diagram depicts using the show i p protocols command to verify that EIGRP routing is enabled. The diagram is based on the network topology described in 9.2.1 diagram 1.
R1#show i p protocols
In the output, the following information is highlighted:
Routing Protocol is e i g r p 1
Distance: internal 90 external 170
The diagram depicts using the show i p protocols command to verify that EIGRP routing is enabled. The diagram is based on the network topology described in 9.2.1 diagram 1.
R1#show i p protocols
In the output, the following information is highlighted:
Routing Protocol is e i g r p 1
Distance: internal 90 external 170
9.2.6 Examining the Routing Table
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Another way to verify that EIGRP and other functions of the router are configured properly is to examine the routing tables with the show ip route command.
Click R1, R2, and R3 in the figure.
By default, EIGRP automatically summarizes routes at the major network boundary. We can disable the automatic summarization with the no auto-summary command, just as we did in RIPv2. We will examine this in more detail in a later section.
Notice that EIGRP routes are denoted in the routing table with a D, which stands for DUAL.
Remember, because EIGRP is a classless routing protocol (includes the subnet mask in the routing update), it supports VLSM and CIDR. We can see in the routing table for R1 that the 172.16.0.0/16 parent network is variably subnetted with three child routes using either a /24 or /30 mask.
9.2.6 - Examining the Routing Table
The diagram depicts using the show i p route command on each router to see the routes that each has learned. EIGRP routes are denoted in the routing table with a D, which stands for DUAL. By default, EIGRP automatically summarizes routes at the major network boundary.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts using the show i p route command on each router to see the routes that each has learned. EIGRP routes are denoted in the routing table with a D, which stands for DUAL. By default, EIGRP automatically summarizes routes at the major network boundary.
The diagram is based on the network topology described in 9.2.1 diagram 1.
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Introducing the Null0 Summary Route
The figure shows the routing table for R2 with two entries highlighted. Notice that EIGRP has automatically included a summary route to Null0 for the classful networks 192.168.10.0/24 and 172.16.0.0/16.
Remember from Chapter 7, "RIPv2," that Null0 is not an actual interface. Notice that the summary routes are sourced from Null0 - this is because these routes are used for advertisement purposes. The 192.168.10.0/24 and 172.16.0.0/16 routes do not actually represent a path to reach the parent networks. If a packet does not match one of the level 2 child routes, it is sent to the Null0 interface. In other words, if the packet matches the level 1 parent - the classful network address - but none of the subnets, the packet is discarded.
Note: EIGRP automatically includes a null0 summary route as a child route whenever both of following conditions exist:
- There is at least one subnet that was learned via EIGRP.
- Automatic summarization is enabled.
9.2.6 - Examining the Routing Table
The diagram depicts the R2 routing table summary routes to null0.
The following R2 summary routes are highlighted:
D 192.168.10.0/24 is a summary, 00:04:13, Null0
D 172.16.0.0/16 is a summary, 00:04:07, Null0
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts the R2 routing table summary routes to null0.
The following R2 summary routes are highlighted:
D 192.168.10.0/24 is a summary, 00:04:13, Null0
D 172.16.0.0/16 is a summary, 00:04:07, Null0
The diagram is based on the network topology described in 9.2.1 diagram 1.
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R3 Routing Table
The routing table for R3 shows that both R1 and R2 are automatically summarizing the 172.16.0.0/16 network and sending it as a single routing update. R1 and R2 are not propagating the individual subnets because of automatic summarization. We will turn off automatic summarization later. Because R3 is getting two equal cost routes for 172.16.0.0/16 from both R1 and R2, both routes are included in the routing table.
9.2.6 - Examining the Routing Table
The diagram depicts the R3 table and shows that both R1 and R2 are automatically summarizing the 172.16.0.0/16 network and sending it as a single routing update to R3. Because R3 is getting two equal-cost routes for 172.16.0.0/16 from R1 and R2, both routes are included in the routing table.
The following equal-cost routes to 172.16.0.0/16 for R3 are highlighted:
D 172.16.0.0/16 [90/2172416] via 192.168.10.5, 00:03:23, Serial0/0/0
[90/2172416] via 192.168.10.9, 00:03:23, Serial0/0/1
The diagram depicts the R3 table and shows that both R1 and R2 are automatically summarizing the 172.16.0.0/16 network and sending it as a single routing update to R3. Because R3 is getting two equal-cost routes for 172.16.0.0/16 from R1 and R2, both routes are included in the routing table.
The following equal-cost routes to 172.16.0.0/16 for R3 are highlighted:
D 172.16.0.0/16 [90/2172416] via 192.168.10.5, 00:03:23, Serial0/0/0
[90/2172416] via 192.168.10.9, 00:03:23, Serial0/0/1
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Use the Packet Tracer Activity to configure and verify basic EIGRP routing.
Click the Packet Tracer icon for more details.
9.2.6 - Examining the Routing Table
Link to Packet Tracer Exploration: Configure and Verify EIGRP Routing
Use the Packet Tracer Activity to configure and verify basic EIGRP routing.
Link to Packet Tracer Exploration: Configure and Verify EIGRP Routing
Use the Packet Tracer Activity to configure and verify basic EIGRP routing.
9.3 EIGRP Metric Calculation
9.3.1 EIGRP Composite Metric and the K ValuesPage 1:
EIGRP uses the following values in its composite metric to calculate the preferred path to a network:
- Bandwidth
- Delay
- Reliability
- Load
The Composite Metric
The figure shows the composite metric formula used by EIGRP. The formula consists of values K1 through K5, known as EIGRP metric weights. By default, K1 and K3 are set to 1, and K2, K4, and K5 are set to 0. The result is that only the bandwidth and delay values are used in the computation of the default composite metric.
The default K values can be changed with the EIGRP router command:
Router(config-router)#metric weights tos k1 k2 k3 k4 k5
Note: Modifying the metric weights is beyond the scope of this course, but their relevance is important in establishing neighbors and is discussed in a later section. The tos (Type of Service) value is left over from IGRP and was never implemented. The tos value is always set to 0.
9.3.1 - EIGRP Composite Metric and the K Values
The diagram depicts the EIGRP composite metric used to calculate the preferred path to a network: The values include:
- Bandwidth
- Delay
- Reliability
- Load
Default Composite Formula:
Metric = [K1 times bandwidth + K3 times delay]
Complete Composite Formula:
Metric = [K1 times bandwidth + (K2 times bandwidth)/(256 - load) + K3 times delay] times [K5/reliability + K4)]
(Not used if K values are 0)
Default values:
K1 (bandwidth) = 1
K2 (load) = 0
K3 (delay) = 1
K4 (reliability) = 0
K5 (reliability) = 0
K values can be changed with the metric weights command.
Router(config-router)#metric weights t o s k1 k2 k3 k4 k5
The diagram depicts the EIGRP composite metric used to calculate the preferred path to a network: The values include:
- Bandwidth
- Delay
- Reliability
- Load
Default Composite Formula:
Metric = [K1 times bandwidth + K3 times delay]
Complete Composite Formula:
Metric = [K1 times bandwidth + (K2 times bandwidth)/(256 - load) + K3 times delay] times [K5/reliability + K4)]
(Not used if K values are 0)
Default values:
K1 (bandwidth) = 1
K2 (load) = 0
K3 (delay) = 1
K4 (reliability) = 0
K5 (reliability) = 0
K values can be changed with the metric weights command.
Router(config-router)#metric weights t o s k1 k2 k3 k4 k5
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Verifying the K Values
The show ip protocols command is used to verify the K values. The command output for R1 is shown in the figure. Notice that the K values on R1 are set to the default. Again, changing these values to other than the default is not recommended unless the network administrator has a very good reason to do so.
9.3.1 - EIGRP Composite Metric and the K Values
The diagram depicts using the show i p protocols command to verify the R1 K values.
The following output line for R1 is highlighted:
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
The diagram depicts using the show i p protocols command to verify the R1 K values.
The following output line for R1 is highlighted:
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
9.3.2 EIGRP Metrics
Page 1:
Examining the Metric Values
You now know the defaults for the K values. By using the show interface command we can examine the actual values used for bandwidth, delay, reliability, and load in the computation of the routing metric.
Click Router Output in the figure.
The output in the figure shows the values used in the composite metric for the Serial 0/0/0 interface on R1.
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Bandwidth
The bandwidth metric (1544 Kbit) is a static value used by some routing protocols such as EIGRP and OSPF to calculate their routing metric. The bandwidth is displayed in Kbit (kilobits). Most serial interfaces use the default bandwidth value of 1544 Kbit or 1,544,000 bps (1.544 Mbps). This is the bandwidth of a T1 connection. However, some serial interfaces use a different default bandwidth value. Always verify bandwidth with the show interface command.
The value of the bandwidth may or may not reflect the actual physical bandwidth of the interface. Modifying the bandwidth value does not change the actual bandwidth of the link. If actual bandwidth of the link differs from the default bandwidth value, then you should modify the bandwidth value, as we will see in a later section.
9.3.2 - EIGRP Metrics
The diagram depicts using the show interface serial 0/0/0 command on R1 to verify metrics. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following output information for R1 is highlighted:
BW 1544 kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255,
Note: usec = microsecond or 1 millionth of a second
The diagram depicts using the show interface serial 0/0/0 command on R1 to verify metrics. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following output information for R1 is highlighted:
BW 1544 kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255,
Note: usec = microsecond or 1 millionth of a second
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Delay
Delay is a measure of the time it takes for a packet to traverse a route. The delay (DLY) metric is a static value based on the type of link to which the interface is connected and is expressed in microseconds. Delay is not measured dynamically. In other words, the router does not actually track how long packets are taking to reach the destination. The delay value, much like the bandwidth value, is a default value that can be changed by the network administrator.
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
The table in the figure shows the default delay values for various interfaces. Notice that the default value is 20,000 microseconds for Serial interfaces and 100 microseconds for FastEthernet interfaces.
9.3.2 - EIGRP Metrics
The diagram depicts delay values in microseconds for various media and network technologies. Fast Ethernet and T1 (Serial Default) are highlighted.
Media: 100M ATM
Delay: 100 microseconds
Media: Fast Ethernet
Delay: 100 microseconds
Media: FDDI
Delay: 100 microseconds
Media: 1HSSI
Delay: 20,000 microseconds
Media: 16M Token Ring
Delay: 630 microseconds
Media: Ethernet
Delay: 1,000 microseconds
Media: T1 (Serial Default)
Delay: 20,000 microseconds
Media: 512K
Delay: 20,000 microseconds
Media: DSO
Delay: 20,000 microseconds
Media: 56K
Delay: 20,000 microseconds
The diagram depicts delay values in microseconds for various media and network technologies. Fast Ethernet and T1 (Serial Default) are highlighted.
Media: 100M ATM
Delay: 100 microseconds
Media: Fast Ethernet
Delay: 100 microseconds
Media: FDDI
Delay: 100 microseconds
Media: 1HSSI
Delay: 20,000 microseconds
Media: 16M Token Ring
Delay: 630 microseconds
Media: Ethernet
Delay: 1,000 microseconds
Media: T1 (Serial Default)
Delay: 20,000 microseconds
Media: 512K
Delay: 20,000 microseconds
Media: DSO
Delay: 20,000 microseconds
Media: 56K
Delay: 20,000 microseconds
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Reliability
Reliability (reliability) is a measure of the probability that the link will fail or how often the link has experienced errors. Unlike delay, Reliability is measured dynamically with a value between 0 and 255, with 1 being a minimally reliable link and 255 one hundred percent reliable. Reliability is calculated on a 5-minute weighted average to avoid the sudden impact of high (or low) error rates.
Reliability is expressed as a fraction of 255 - the higher the value, the more reliable the link. So, 255/255 would be 100 percent reliable, whereas a link of 234/255 would be 91.8 percent reliable.
Remember: By default, EIGRP does not use reliability in its metric calculation.
Load
Load (load) reflects the amount of traffic utilizing the link. Like reliability, load is measured dynamically with a value between 0 and 255. Similar to reliability, load is expressed as a fraction of 255. However, in this case a lower load value is more desirable because it indicates less load on the link. So, 1/255 would be a minimally loaded link. 40/255 is a link at 16 percent capacity, and 255/255 would be a link that is 100 percent saturated.
Load is displayed as both an outbound, or transmit, load value (txload) and an inbound, or receive, load value (rxload). This value is calculated on a 5-minute weighted average to avoid the sudden impact of high (or low) channel usage.
Remember: By default, EIGRP does not use load in its metric calculation.
9.3.2 - EIGRP Metrics
The diagram depicts reliability and load values in the output of the show interface to serial 0/0/0 command.
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 microseconds,
reliability 255/255, txload 1/255, rxload 1/255
Reliability value: reliability 255/255
Load value: txload 1/255, rxload 1/255
The diagram depicts reliability and load values in the output of the show interface to serial 0/0/0 command.
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 microseconds,
reliability 255/255, txload 1/255, rxload 1/255
Reliability value: reliability 255/255
Load value: txload 1/255, rxload 1/255
9.3.3 Using the bandwidth Command
Page 1:
On most serial links, the bandwidth metric will default to 1544 Kbits. Because both EIGRP and OSPF use bandwidth in default metric calculations, a correct value for bandwidth is very important to the accuracy of routing information. But what do you do if the actual bandwidth of the link does not match the default bandwidth of the interface?
Click Configure Bandwidth in the figure.
Use the interface command bandwidth to modify the bandwidth metric:
Router(config-if)#bandwidth kilobits
Use the interface command no bandwidth to restore the default value.
In the figure, the link between R1 and R2 has a bandwidth of 64 kbps, and the link between R2 and R3 has a bandwidth of 1024 kbps. The figure shows the configurations used on all three routers to modify the bandwidth on the appropriate serial interfaces.
Click Verify Bandwidth in the figure.
We can verify the change using the show interface command. It is important to modify the bandwidth metric on both sides of the link to ensure proper routing in both directions.
Note: A common misconception for students new to networking and the Cisco IOS is to assume that the bandwidth command will change the physical bandwidth of the link. As stated in the previous section, the bandwidth command only modifies the bandwidth metric used by routing protocols such as EIGRP and OSPF. Sometimes, a network administrator will change the bandwidth value in order have more control over the chosen outgoing interface.
9.3.3 - Using the bandwidth Command
The diagram depicts configuring bandwidth using the bandwidth command and verifying bandwidth using the show interface serial 0/0/0 command.
Configuring bandwidth for R1:
R1(config)#interface serial 0/0/0
R1(config-i f)#bandwidth 64
Configuring bandwidth for R2:
R2(config)#interface serial 0/0/0
R2(config-i f)#bandwidth 64
R2(config)#interface serial 0/0/1
R2(config-i f)#bandwidth 1024
Configuring bandwidth for R3:
R3(config)#interface serial 0/0/0
R3(config-i f)#bandwidth 1024
Note: The actual bandwidth of the link between R1 and R3 matches the default value for serial interface (1544 kbps). Therefore, the bandwidth command is not required.
Verify Bandwidth:
The R2 show interface serial 0/0/0 command shows the following highlighted information:
BW 64 Kbit.
The R2 show interface serial 0/0/1 command shows the following highlighted:
BW 1024 Kbit.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts configuring bandwidth using the bandwidth command and verifying bandwidth using the show interface serial 0/0/0 command.
Configuring bandwidth for R1:
R1(config)#interface serial 0/0/0
R1(config-i f)#bandwidth 64
Configuring bandwidth for R2:
R2(config)#interface serial 0/0/0
R2(config-i f)#bandwidth 64
R2(config)#interface serial 0/0/1
R2(config-i f)#bandwidth 1024
Configuring bandwidth for R3:
R3(config)#interface serial 0/0/0
R3(config-i f)#bandwidth 1024
Note: The actual bandwidth of the link between R1 and R3 matches the default value for serial interface (1544 kbps). Therefore, the bandwidth command is not required.
Verify Bandwidth:
The R2 show interface serial 0/0/0 command shows the following highlighted information:
BW 64 Kbit.
The R2 show interface serial 0/0/1 command shows the following highlighted:
BW 1024 Kbit.
The diagram is based on the network topology described in 9.2.1 diagram 1.
9.3.4 Calculating the EIGRP Metric
Page 1:
The figure shows the composite metric used by EIGRP. Using the default values for K1 and K3, we can simplify this calculation to: the slowest bandwidth (or minimum bandwidth) plus the cumulative sum of all of the delays.
In other words, by examining the bandwidth and delay values for all of the outgoing interfaces of the route, we can determine the EIGRP metric. First, determine the link with the slowest bandwidth. That bandwidth is used for the (10,000,000/bandwidth) * 256 portion of the formula. Next, determine the delay value for each outgoing interface on the way to the destination. Sum the delay values and divide by 10 (sum of delay/10) and then multiply by 256 (* 256). Add the bandwidth and sum of delay values to obtain the EIGRP metric.
The routing table output for R2 shows that the route to 192.168.1.0/24 has an EIGRP metric of 3,014,400. Let's see exactly how EIGRP calculated this value.
9.3.4 - Calculating the EIGRP Metric
The diagram depicts an example of calculating the EIGRP default metric.
Default metric = [K1 times bandwidth + K3 times delay] times 256
Because K1 and K3 both equal 1, the formula simplifies to bandwidth + delay
bandwidth = sum of the slowest link in the route to the destination
delay = sum of the delays of each link in the route to the destination
Other text:
Slowest Bandwidth: (10,000,000/bandwidth kbps) times 256 plus the sum of the delays: + (sum of delay/10) times 256 is equal to the EIGRP metric.
In the show i p route output for R2, the EIGRP calculated metric of 3014400 is highlighted:
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:02:14, Serial0/0/1
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts an example of calculating the EIGRP default metric.
Default metric = [K1 times bandwidth + K3 times delay] times 256
Because K1 and K3 both equal 1, the formula simplifies to bandwidth + delay
bandwidth = sum of the slowest link in the route to the destination
delay = sum of the delays of each link in the route to the destination
Other text:
Slowest Bandwidth: (10,000,000/bandwidth kbps) times 256 plus the sum of the delays: + (sum of delay/10) times 256 is equal to the EIGRP metric.
In the show i p route output for R2, the EIGRP calculated metric of 3014400 is highlighted:
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:02:14, Serial0/0/1
The diagram is based on the network topology described in 9.2.1 diagram 1.
Page 2:
Bandwidth
Click Bandwidth Calculation in the figure.
Because EIGRP uses the slowest bandwidth in its metric calculation, we can find the slowest bandwidth by examining each interface between R2 and the destination network 192.168.1.0. The Serial 0/0/1 interface on R2 has a bandwidth of 1,024 Kbps or 1,024,000 bps. The FastEthernet 0/0 interface on R3 has a bandwidth of 100,000 Kbps or 100 Mbps. Therefore, the slowest bandwidth is 1024 Kbps and is used in the calculation of the metric.
EIGRP takes the reference bandwidth value of 10,000,000 and divides it by the bandwidth value in kbps. This will result in higher bandwidth values receiving a lower metric and lower bandwidth values receiving a higher metric.
10,000,000 is divided by 1024. If the result is not a whole number, then the value is rounded down. In this case, 10,000,000 divided by 1024 equals 9765.625. The .625 is dropped before multiplying by 256. The bandwidth portion of the composite metric is 2,499,840.
Delay
Using the same outgoing interfaces we can also determine the delay value.
Click Delay Calculation in the figure.
EIGRP uses the cumulative sum of delay metrics of all of the outgoing interfaces. The Serial 0/0/1 interface on R2 has a delay of 20000 microseconds. The FastEthernet 0/0 interface on R3 has a delay of 100 microseconds.
Each delay value is divided by 10 and then summed. 20,000/10 + 100/10 results in a value of 2,010. This result is then multiplied by 256. The delay portion of the composite metric is 514,560.
Adding Bandwidth and Delay
Click EIGRP Metric in the figure.
Simply add the two values together, 2,499,840 + 514,560, to obtain the EIGRP metric of 3,014,400. This value matches the value shown in the routing table for R2. This is a result of the slowest bandwidth and the sum of the delays
9.3.4 - Calculating the EIGRP Metric
The diagram depicts finding the slowest bandwidth using the show interface serial 0/0/0 command.
The output from R2 shows the following highlighted information:
R2#show interface serial 0/0/1
BW 1024 Kbit, DLY 20000 usec
R3#show interface FA0/0
BW 100000 Kbit, DLY 100 usec,
Bandwidth = (10,000,000/1024) = 9765 times 256 = 2,499,840
Delay = [(20000/10) + (100/10)] times 256 = 514,560
EIGRP Metric = bandwidth + delay = 2,499,840 + 514,560 = 3,014,400.
This value matches the value shown in the routing table for R2. This is a result of the slowest bandwidth and the sum of the delays.
The diagram depicts finding the slowest bandwidth using the show interface serial 0/0/0 command.
The output from R2 shows the following highlighted information:
R2#show interface serial 0/0/1
BW 1024 Kbit, DLY 20000 usec
R3#show interface FA0/0
BW 100000 Kbit, DLY 100 usec,
Bandwidth = (10,000,000/1024) = 9765 times 256 = 2,499,840
Delay = [(20000/10) + (100/10)] times 256 = 514,560
EIGRP Metric = bandwidth + delay = 2,499,840 + 514,560 = 3,014,400.
This value matches the value shown in the routing table for R2. This is a result of the slowest bandwidth and the sum of the delays.
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Use the Packet Tracer Activity to investigate EIGRP's metric calculations.
Click the Packet Tracer icon for more details.
9.3.4 - Calculating the EIGRP Metric
Link to Packet Tracer Exploration: Calculating the EIGRP metric
Use the Packet Tracer Activity to investigate EIGRP's metric calculations.
Link to Packet Tracer Exploration: Calculating the EIGRP metric
Use the Packet Tracer Activity to investigate EIGRP's metric calculations.
9.4 DUAL
9.4.1 DUAL ConceptsPage 1:
As stated in a previous section, DUAL (Diffusing Update Algorithm) is the algorithm used by EIGRP. This section will discuss how DUAL determines the best loop-free path and loop-free backup paths.
DUAL uses several terms which will be discussed in more detail throughout this section:
- Successor
- Feasible Distance (FD)
- Feasible Successor (FS)
- Reported Distance (RD) or Advertised Distance (AD)
- Feasible Condition or Feasibility Condition (FC)
9.4.1 - DUAL Concepts
The diagram depicts a listing of DUAL characteristics.
DUAL provides:
- Loop-free paths
- Loop-free backup paths that can be used immediately
- Fast convergence
- Minimum bandwidth usage with bounded updates
The diagram depicts a listing of DUAL characteristics.
DUAL provides:
- Loop-free paths
- Loop-free backup paths that can be used immediately
- Fast convergence
- Minimum bandwidth usage with bounded updates
9.4.2 Successor and Feasible Distance
Page 1:
A successor is a neighboring router that is used for packet forwarding and is the least-cost route to the destination network. The IP address of a successor is shown in a routing table entry right after the word via.
Feasible distance (FD) is the lowest calculated metric to reach the destination network. FD is the metric listed in the routing table entry as the second number inside the brackets. As with other routing protocols this is also known as the metric for the route.
Click Router Output in the figure.
Examining the routing table for R2 in the figure, we can see that EIGRP's best path for the 192.168.1.0/24 network is through router R3 and that the feasible distance is 3014400-the same metric that we calculated in the last topic:
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:00:31, Serial0/0/1
Other successors and feasible distances are also shown in the figure. Can you answer the following questions?
What is the IP address of the successor for network 172.16.1.0/24?
Answer: 172.16.3.1, which is R1.
What is the feasible distance to 172.16.1.0/24?
Answer: 40514560.
9.4.2 - Successor and Feasible Distance
The diagram depicts feasible distance and successor in the highlighted output of the show i p route command.
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:00:31, Serial0/0/1
In the output, R3 at 192.168.10.10 is the successor for network 192.168.1.0/24. This route has a feasible distance of 3014400.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts feasible distance and successor in the highlighted output of the show i p route command.
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:00:31, Serial0/0/1
In the output, R3 at 192.168.10.10 is the successor for network 192.168.1.0/24. This route has a feasible distance of 3014400.
The diagram is based on the network topology described in 9.2.1 diagram 1.
9.4.3 Feasible Successors, Feasibility Condition and Reported Distance
Page 1:
One of the reasons DUAL can converge quickly after a change in the topology is because it can use backup paths to other routers known as feasible successors without having to recompute DUAL.
Click Feasible Successor in the figure.
A feasible successor (FS) is a neighbor who has a loop-free backup path to the same network as the successor by satisfying the feasibility condition. In our topology, would R2 consider R1 to be a feasible successor to network 192.168.1.0/24? In order to be a feasible successor, R1 must satisfy the feasibility condition (FC). Let's examine what that means.
Click Feasibility Condition in the figure.
The feasibility condition (FC) is met when a neighbor's reported distance (RD) to a network is less than the local router's feasible distance to the same destination network. The reported distance or advertised distance is simply an EIGRP neighbor's feasible distance to the same destination network. The reported distance is the metric that a router reports to a neighbor about its own cost to that network.
If R3 is the successor, can the neighbor R1 be a feasible successor to this same 192.161.0/24 network? In other words, if the link between R2 and R3 fails can R1 immediately be used as a backup path without a recomputation of the DUAL algorithm? R1 can only be a feasible successor if it meets the feasibility condition.
In the figure, R1 is reporting to R2 that its feasible distance to 192.168.1.0/24 is 2172416. From R2's perspective, 2172416 is R1's reported distance. From R1's perspective, 2172416 is its feasible distance.
Click Reported Distance in the figure.
R2 examines the reported distance (RD) of 2172416 from R1. Because the reported distance (RD) of R1 is less than R2's own feasible distance (FD) of 3014400, R1 meets the feasibility condition. R1 is now a feasible successor for R2 to the 192.168.1.0/24 network.
Why isn't R1 the successor if its reported distance (RD) is less than R2's feasible distance (FD) to 192.168.1.0/24? Because the total cost for R2, its feasible distance (FD), to reach 192.168.1.0/24 is greater through R1 than it is through R3.
9.4.3 - Feasible Successors, Feasibility Condition, and Reported Distance
The diagram depicts finding the feasible successor. In the show i p route command output for R2, the feasible successor is 192.168.10.10, which is the route to R3.
The feasibility condition says R1 reports to R2 that its feasible distance to 192.168.1.0/24 is 2172416.
R2 examines the reported distance (RD) of 2172416 from R1. Because the reported distance of R1 is less than R2's own feasible distance (FD) of 3014400, R1 meets the feasibility condition. R1 is now a feasible successor for R2 to the 192.168.1.0/24 network.
The diagram is based on the network topology described in 9.2.1 diagram 1, except that the WAN link between R2 and R3 is down.
The diagram depicts finding the feasible successor. In the show i p route command output for R2, the feasible successor is 192.168.10.10, which is the route to R3.
The feasibility condition says R1 reports to R2 that its feasible distance to 192.168.1.0/24 is 2172416.
R2 examines the reported distance (RD) of 2172416 from R1. Because the reported distance of R1 is less than R2's own feasible distance (FD) of 3014400, R1 meets the feasibility condition. R1 is now a feasible successor for R2 to the 192.168.1.0/24 network.
The diagram is based on the network topology described in 9.2.1 diagram 1, except that the WAN link between R2 and R3 is down.
9.4.4 Topology Table: Successor and Feasible Successor
Page 1:
The successor, feasible distance, and any feasible successors with their reported distances are kept by a router in its EIGRP topology table or topology database. As shown in the figure, the topology table can be viewed using the show ip eigrp topology command. The topology table lists all successors and feasible successors that DUAL has calculated to destination networks.
9.4.4 - Topology Table: Successor and Feasible Successor
The diagram depicts using the show i p e i g r p topology command to display the EIGRP topology table. The topology table lists all successors and feasible successors that DUAL has calculated to destination networks.
The following output for R2 from the show i p e i g r p topology command is highlighted.
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
There are two possible routes for 192.168.1.0/24.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts using the show i p e i g r p topology command to display the EIGRP topology table. The topology table lists all successors and feasible successors that DUAL has calculated to destination networks.
The following output for R2 from the show i p e i g r p topology command is highlighted.
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
There are two possible routes for 192.168.1.0/24.
The diagram is based on the network topology described in 9.2.1 diagram 1.
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Click Play to view the animation.
A detailed description of each part of the topology table entry for destination network 192.168.1.0/24 appears below.
The first line displays:
- P - This route is in the passive state. When DUAL is not performing its diffusing computations to determine a path for a network, the route will be in a stable mode, known as the passive state. If DUAL is recalculating or searching for a new path, the route will be in an active state. All routes in the topology table should be in the passive state for a stable routing domain. DUAL will display an A if the route is "Active," which is a CCNP-level troubleshooting issue.
- 192.168.1.0/24 - This is the destination network that is also found in the routing table.
- 1 successors - This shows the number of successors for this network. If there are multiple equal cost paths to this network, there will be multiple successors.
- FD is 3014400 - This is the feasible distance, the EIGRP metric to reach the destination network.
- via 192.168.10.10 - This is the next-hop address of the successor, R3. This address is shown in the routing table.
- 3014400 - This is the feasible distance to 192.168.1.0/24. It is the metric shown in the routing table.
- 28160 - This is the reported distance of the successor and is R3's cost to reach this network.
- Serial0/0/1 - This is the outbound interface used to reach this network, also shown in the routing table.
- via 172.16.3.1 - This is the next-hop address of the feasible successor, R1.
- 41026560 - This would be R2's new feasible distance to 192.168.1.0/24 if R1 became the new successor.
- 2172416 - This is the reported distance of the feasible successor or R1's metric to reach this network. This value, RD, must be less than the current FD of 3014400 to meet the feasibility condition.
- Serial0/0/0 - This is the outbound interface used to reach feasible successor, if this router becomes the successor.
9.4.4 - Topology Table: Successor and Feasible Successor
The animation depicts an explanation of the components for the R2 table entry for 192.168.1.0/24.
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
P = Passive; DUAL is not computing a new path.
192.168.1.0/24 = Destination Network
1 successors = Number of Successors
3014400 - Feasible Distance to the Destination Network
28160 = Successor's Reported Distance
192.168.10.10 = Next-Hop Address for Successor
172.16.3.1 = Next-Hop Address for Feasible Successor
41026560 = Feasible Distance to Destination Network
2172416 = Feasible Successor's Reported Distance
Serial0/0/1 and Serial0/0/0 = Outbound Interfaces to Reach Network
The animation depicts an explanation of the components for the R2 table entry for 192.168.1.0/24.
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
P = Passive; DUAL is not computing a new path.
192.168.1.0/24 = Destination Network
1 successors = Number of Successors
3014400 - Feasible Distance to the Destination Network
28160 = Successor's Reported Distance
192.168.10.10 = Next-Hop Address for Successor
172.16.3.1 = Next-Hop Address for Feasible Successor
41026560 = Feasible Distance to Destination Network
2172416 = Feasible Successor's Reported Distance
Serial0/0/1 and Serial0/0/0 = Outbound Interfaces to Reach Network
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To view detailed information about the metrics of a specific entry in the topology table, add the optional parameter [network] to the show ip eigrp topology command, as shown in the figure:
R2#show ip eigrp topology 192.168.1.0
Remember that EIGRP is a distance vector routing protocol. This command lists the full list of distance vector metrics available to EIGRP even though, by default, EIGRP only uses bandwidth and delay. It also displays other information included in the routing update, but not included in the composite metric: minimum MTU and hop count.
9.4.4 - Topology Table: Successor and Feasible Successor
The diagram depicts using the show i p e i g r p topology 192.168.1.0 command to view detailed information about the metrics of a specific entry in the topology table.
R2#show i p e i g r p topology 192.168.1.0
The following is highlighted information in the output.
Path via 192.168.10.10:
Minimum bandwidth is 1024 Kbit
Total delay is 20100 microseconds
Path via 172.16.3.1:
Minimum bandwidth is 64 Kbit
Total delay is 40100 microseconds
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts using the show i p e i g r p topology 192.168.1.0 command to view detailed information about the metrics of a specific entry in the topology table.
R2#show i p e i g r p topology 192.168.1.0
The following is highlighted information in the output.
Path via 192.168.10.10:
Minimum bandwidth is 1024 Kbit
Total delay is 20100 microseconds
Path via 172.16.3.1:
Minimum bandwidth is 64 Kbit
Total delay is 40100 microseconds
The diagram is based on the network topology described in 9.2.1 diagram 1.
9.4.5 Topology Table: No Feasible Successor
Page 1:
To continue our understanding of DUAL and its use of successors and feasible successors, let's look at the routing table for R1.
Click R1 Routing Table in the figure.
The route to 192.168.1.0/24 shows that the successor is R3 via 192.168.10.6 with a feasible distance of 2172416.
D 192.168.1.0/24 [90/2172416] via 192.168.10.6, 00:56:13, Serial0/1
Now let's examine the topology table to see if there are any feasible successors for this route.
Click R1 Topology Table in the figure.
The topology table only shows the successor 192.168.10.6. There are no feasible successors. By looking at the actual physical topology or network diagram, it is obvious that there is a backup route to 192.168.1.0/24 through R2. Why isn't R2 listed as a feasible successor? R2 is not a feasible successor because it does not meet the feasibility condition.
Although, looking at the topology it is obvious that R2 is a backup route, EIGRP does not have a map of the network topology. EIGRP is a distance vector routing protocol and only knows about remote network information through its neighbors.
Therefore, DUAL does not store the route through R2 in the topology table. However, we can view all possible links whether they satisfy the feasible condition or not by adding the [all-links] option to the show ip eigrp topology command.
Click R1 Topology Table [all-links] in the figure.
The show ip eigrp topology all-links command shows all possible paths to a network including successors, feasible successors, and even those routes that are not feasible successors. R1's feasible distance to 192.168.1.0/24 is 2172416 via the successor R3. For R2 to be considered a feasible successor, it must meet the feasibility condition. R2's feasible distance to reach 192.168.1.0/24 must be less the R1's current feasible distance (FD). As we can see in the figure, R2's feasible distance is 3014400, which is higher than R1's feasible distance of 2172416.
Even though R2 looks like a viable backup path to 192.168.1.0/24, R1 has no idea that its path is not a potential loop back through itself. EIGRP is a distance vector routing protocol, without the ability to see a complete, loop-free topological map of the network. DUAL's method of guaranteeing that a neighbor has a loop-free path is that the neighbor's metric must satisfy the feasibility condition. By ensuring that the RD of the neighbor is less than its own FD, the router can assume that this neighboring router is not part of its own advertised route, thus always avoiding the potential for a loop.
Does this mean R2 cannot be used if the successor fails? No, R3 can be used, but there will be a longer delay before adding it to the routing table. Before this can happen, DUAL will need to do some further processing, which is explained in the next topic.
9.4.5 - Topology Table: No Feasible Successor
The diagram depicts R1 routing and topology tables and it can be seen that they do not have a feasible successor route to 192.168.1.0.
In the output from the show i p route command, the following is highlighted.
D 192.168.1.0/24 [90/2172416] via 192.168.10.6, 00:56:13, Serial0/0/1
The output from the R1 show i p e i g r p topology command shows that there is a successor route, but no feasible successors for this route.
The following R1 command output is highlighted:
P 192.168.1.0/24, 1 successors, FD is 2172416
via 192.168.10.6 (2172416/28160), Serial0/0/1
The output from the R1 show i p e i g r p topology all-links command shows that there is a route from R1, but not one that meets the feasibility condition. The RD from R2 is higher than the FD to R1.
P 192.168.1.0/24, 1 successors, FD is 2172416, serno 5
via 192.168.10.6 (2172416/28160), Serial0/0/1
via 172.16.3.2 (14026560/3014400), Serial0/0/0
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts R1 routing and topology tables and it can be seen that they do not have a feasible successor route to 192.168.1.0.
In the output from the show i p route command, the following is highlighted.
D 192.168.1.0/24 [90/2172416] via 192.168.10.6, 00:56:13, Serial0/0/1
The output from the R1 show i p e i g r p topology command shows that there is a successor route, but no feasible successors for this route.
The following R1 command output is highlighted:
P 192.168.1.0/24, 1 successors, FD is 2172416
via 192.168.10.6 (2172416/28160), Serial0/0/1
The output from the R1 show i p e i g r p topology all-links command shows that there is a route from R1, but not one that meets the feasibility condition. The RD from R2 is higher than the FD to R1.
P 192.168.1.0/24, 1 successors, FD is 2172416, serno 5
via 192.168.10.6 (2172416/28160), Serial0/0/1
via 172.16.3.2 (14026560/3014400), Serial0/0/0
The diagram is based on the network topology described in 9.2.1 diagram 1.
9.4.6 Finite State Machine
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DUAL Finite State Machine (FSM)
The centerpiece of EIGRP is DUAL and its EIGRP route-calculation engine. The actual name of this technology is DUAL Finite State Machine (FSM). This finite state machine contains all of the logic used to calculate and compare routes in an EIGRP network. The figure shows a simplified version of the DUAL FSM.
A finite state machine is an abstract machine, not a mechanical device with moving parts. FSMs define a set of possible states that something can go through, what events cause those states, and what events result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm will react to a set of input events. Finite state machines are beyond the scope of this course; however, we introduce the concept in order to examine some of the output from EIGRP's finite state machine using debug eigrp fsm. Let's use the command to watch what DUAL does when a route is removed from the routing table.
9.4.6 - Finite State Machine
The diagram depicts a flowchart of how the DUAL finite state machine functions when connectivity to a successor is lost.
1. Lost connectivity to successor.
2. Is there a feasible successor?
If yes,
- Promote to successor.
- Install successor in routing table.
- Select new successor.
- Install feasible successors, if any, in the topology table.
If no:
- Place destination network in active state.
- Query neighbors for new route.
3. Are there one or more new routes?
If yes,
- Install successor in routing table.
If no:
- Remove destination network from topology and routing tables.
The diagram depicts a flowchart of how the DUAL finite state machine functions when connectivity to a successor is lost.
1. Lost connectivity to successor.
2. Is there a feasible successor?
If yes,
- Promote to successor.
- Install successor in routing table.
- Select new successor.
- Install feasible successors, if any, in the topology table.
If no:
- Place destination network in active state.
- Query neighbors for new route.
3. Are there one or more new routes?
If yes,
- Install successor in routing table.
If no:
- Remove destination network from topology and routing tables.
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Click R2 Topology Table 1 in the figure.
Remember from our previous discussions that R2 is currently using R3 as the successor to 192.168.1.0/24. In addition, R2 currently lists R1 as a feasible successor. Let's watch what happens when we simulate a failure of the link between R2 and R3.
Click R2 Debug Output in the figure.
First, we turn on DUAL debugging with the debug eigrp fsm command. Then, we simulate a link failure using the shutdown command on the Serial 0/0/1 interface on R2.
When you do this on a real router or Packet Tracer, you will see all the activity generated by DUAL when a link goes down. R2 must inform all EIGRP neighbors of the lost link as well as take care of updating its own routing and topology tables. The figure in this example only shows selected debug output. In particular, notice that the DUAL finite state machine searches for and finds a feasible successor for the route in the EIGRP topology table. The feasible successor, R1, now becomes the successor and is installed in the routing table as the new best path to 192.168.1.0/24.
Click R2 Topology Table 2 in the figure.
The topology table for R2 now shows R1 as the successor and there are no new feasible successors.
If you are following along on routers or Packet Tracer, be sure to restore the original topology by re-activating the Serial 0/0/1 interface on R2 with the no shutdown command.
9.4.6 - Finite State Machine
The diagram depicts what happens when we simulate a failure of the link between R2 and R3, and R2 selects an alternate route through a feasible successor.
Highlighted output to network 192.168.1.0 from the R2 topology table 1.
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
Note: R3 is the successor. R1 is a feasible successor.
The debug e i g r p fsm command on R2 shows that the DUAL finite state machine searches for and finds a feasible successor for the route in the EIGRP topology table. The feasible successor, R1, now becomes the successor and is installed in the routing table as the new best path to 192.168.1.0/24.
Highlighted output to network 192.168.1.0 from the R2 topology table 2.
via 172.16.3.1(41026560/2172416), Serial0/0/0
Note: R1 is now the successor.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts what happens when we simulate a failure of the link between R2 and R3, and R2 selects an alternate route through a feasible successor.
Highlighted output to network 192.168.1.0 from the R2 topology table 1.
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
Note: R3 is the successor. R1 is a feasible successor.
The debug e i g r p fsm command on R2 shows that the DUAL finite state machine searches for and finds a feasible successor for the route in the EIGRP topology table. The feasible successor, R1, now becomes the successor and is installed in the routing table as the new best path to 192.168.1.0/24.
Highlighted output to network 192.168.1.0 from the R2 topology table 2.
via 172.16.3.1(41026560/2172416), Serial0/0/0
Note: R1 is now the successor.
The diagram is based on the network topology described in 9.2.1 diagram 1.
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No Feasible Successor
What if the path to the successor fails and there are no feasible successors? Remember, just because DUAL does not have a feasible successor does not mean that there is not another path to the network. It just means that DUAL does not have a guaranteed loop-free backup path to the network, so it wasn't added to the topology table as a feasible successor. If there are no feasible successors in the topology table, DUAL will put the network into the active state. DUAL will actively query its neighbors for a new successor.
Click R1 Topology Table 1 in the figure.
R1 is currently using R3 as the successor to 192.168.1.0/24. However, R1 does not have R2 listed as a feasible successor because R2 does not satisfy the feasibility condition. Let's watch what happens when we simulate a failure of the link between R1 and R3.
Click R1 Debug Output in the figure.
First, we turn on DUAL debugging with the debug eigrp fsm command. Then, we simulate a link failure using the shutdown command on the Serial 0/0/1 interface on R1.
The selected debug output shows the 192.168.1.0/24 network put into the active state and EIGRP queries are sent to other neighbors. R2 replies with a path to this network, which becomes the new successor and is installed into the routing table.
When the successor is no longer available and there is no feasible successor, DUAL will put the route into active state. DUAL will send EIGRP queries asking other routers for a path to this network. Other routers will return EIGRP replies, letting the sender of the EIGRP query know whether or not they have a path to the requested network. If none of the EIGRP replies have a path to this network, the sender of the query will not have a route to this network.
If the sender of the EIGRP queries receives EIGRP replies that include a path to the requested network, the preferred path is added as the new successor and added to the routing table. This process will take longer than if DUAL had a feasible successor in its topology table and was able to quickly add the new route to the routing table.
Note: DUAL FSM and the process of queries and replies is beyond the scope of this course.
Click R1 Topology Table 2 in the figure.
The topology table for R1 now shows R2 as the successor and there are no new feasible successors.
If you are following along on routers or Packet Tracer, be sure to restore the original topology by re-activating the Serial 0/0/1 interface on R1 with the no shutdown command.
9.4.6 - Finite State Machine
The diagram depicts what happens to the R1 topology tables and debug output when R1 does not have a feasible successor route to 192.168.1.0.
R1 Topology Table 1:
via 192.168.10.6 (2172416/28160), Serial0/0/1
R1 is currently using R3 as the successor to 192.168.1.0/24.
R1 Debug Output:
The debug output shows the 192.168.1.0/24 network put into the active state, and EIGRP queries are sent to other neighbors. R2 replies with a path to this network, which becomes the new successor and is installed into the R1 routing table.
R1 Topology Table 2:
via 172.16.3.2 (41026560/3014400), Serial0/0/0
R1 now shows R2 as the successor and that there are no new feasible successors.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts what happens to the R1 topology tables and debug output when R1 does not have a feasible successor route to 192.168.1.0.
R1 Topology Table 1:
via 192.168.10.6 (2172416/28160), Serial0/0/1
R1 is currently using R3 as the successor to 192.168.1.0/24.
R1 Debug Output:
The debug output shows the 192.168.1.0/24 network put into the active state, and EIGRP queries are sent to other neighbors. R2 replies with a path to this network, which becomes the new successor and is installed into the R1 routing table.
R1 Topology Table 2:
via 172.16.3.2 (41026560/3014400), Serial0/0/0
R1 now shows R2 as the successor and that there are no new feasible successors.
The diagram is based on the network topology described in 9.2.1 diagram 1.
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Use the Packet Tracer Activity to investigate successors and feasible successors as well as watch the DUAL FSM remove and install routes.
Click the Packet Tracer icon for more details.
9.4.6 - Finite State Machine
Link to Packet Tracer Exploration: Finite State Machine
Use the Packet Tracer Activity to investigate successors and feasible successors as well as watch the DUAL FSM remove and install routes.
Link to Packet Tracer Exploration: Finite State Machine
Use the Packet Tracer Activity to investigate successors and feasible successors as well as watch the DUAL FSM remove and install routes.
9.5 More EIGRP Configurations
9.5.1 The Null0 Summary RoutePage 1:
Analyzing a routing table containing EIGRP routes can be confusing due to EIGRP's automatic inclusion of Null0 summary routes. In the figure, R1's routing table contains two routes that have an exit interface of Null0. Remember from Chapter 7, "RIPv2," that the Null0 interface is simply a route to nowhere, commonly known as "the bit bucket." So by default, EIGRP uses the Null0 interface to discard any packets that match the parent route but do not match any of the child routes.
You might think that if we configure classless routing behavior with the ip classless command, EIGRP would not discard that packet but would continue looking for a default or supernet route. However, the EIGRP Null0 summary route is a child route that will match any possible packets of the parent route that do not match another child route. Even with classless routing behavior, ip classless, where you would expect the route lookup process to check for supernets and default routes, EIGRP will use the Null0 summary route and discard the packet because this route will match any packets of the parent that do not have a child route.
Regardless of whether classful or classless routing behavior is being used, the null0 summary will be used and therefore denying the use of any supernet or default route.
In the figure, R1 will discard any packets that match the parent 172.16.0.0/16 classful network but do not match one of the child routes 172.16.1.0/24, 172.16.2.0/24 or 172.16.3.0/24. For example, a packet to 172.16.4.10 would be discarded. Even if a default route was configured, R1 would still discard the packet because it matches the Null0 summary route to 172.16.0.0/16.
D 172.16.0.0/16 is a summary, 00:46:10, Null0
Note: EIGRP automatically includes a null0 summary route as a child route whenever both of following conditions exist:
- There is at least one subnet that was learned via EIGRP.
- Automatic summarization is enabled.
9.5.1 - The Null0 Summary Route
The diagram depicts examples of EIGRP's automatic inclusion of Null0 summary routes. The diagram is based on the network topology described in 9.2.1 diagram 1.
In the output of the R1 show i p route command, the following summary routes are highlighted.
192.168.10.0/24 is a summary, 00:45:09, Null0
172.16.0.0/16 is a summary, 00:46:10, Null0
EIGRP installs a Null0 summary route for each parent route. Packets matching the Null0 summary route are discarded.
The diagram depicts examples of EIGRP's automatic inclusion of Null0 summary routes. The diagram is based on the network topology described in 9.2.1 diagram 1.
In the output of the R1 show i p route command, the following summary routes are highlighted.
192.168.10.0/24 is a summary, 00:45:09, Null0
172.16.0.0/16 is a summary, 00:46:10, Null0
EIGRP installs a Null0 summary route for each parent route. Packets matching the Null0 summary route are discarded.
9.5.2 Disabling Automatic Summarization
Page 1:
Like RIP, EIGRP automatically summarizes at major network boundaries using the default auto-summary command. We can see the result of this by looking at the routing table for R3.
Click R3 Routing Table in the figure.
Notice that R3 is not receiving individual routes for the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 subnets. Both R1 and R2 automatically summarized those subnets to the 172.16.0.0/16 classful boundary when sending EIGRP update packets to R3. The result is that R3 has one route to 172.16.0.0/16 through R1. R1 is the successor because of the difference in bandwidth.
D 172.16.0.0/16 [90/2172416] via 192.168.10.5, 01:08:30, Serial0/0/0
You can quickly see that this route is not optimal. R3 will route all packets destined for 172.16.2.0 through R1. R3 does not know that R1 will then have to route these packets across a very slow link to R2. The only way R3 can learn about this slow bandwidth is if R1 and R2 send individual routes for each of the 172.16.0.0/16 subnets. In other words, R1 and R2 must stop automatically summarizing 172.16.0.0/16.
Click no auto-summary in the figure.
As in RIPv2, automatic summarization can be disabled with the no auto-summary command. The router configuration command eigrp log-neighbor-changes is on by default on some IOS implementations. If on, you will see output similar to that shown for R1. DUAL takes down all neighbor adjacencies and then reestablishes them so that the effect of the no auto-summary command can be fully realized. All EIGRP neighbors will immediately send out a new round of updates that will not be automatically summarized.
Click R1, R2, and R3 in the figure.
We can see in the routing tables for all three routers that EIGRP is now propagating individual subnets. Notice that EIGRP no longer includes the Null0 summary route, because automatic summarization has been disabled with no auto-summary. As long as the default classless routing behavior (ip classless) is in effect, supernet and default routes will be used when there is not a match with a subnet route.
9.5.2 - Disabling Automatic Summarization
The diagram depicts EIGRP automatically summarizing at major network boundaries using the default auto-summary command. This is shown in the routing table for R3, where a single 172.16.0.0/16 classful route is found.
D 172.16.0.0/16 [90/2172416] via 192.168.10.5, 01:08:30, Serial0/0/0
R3 uses this suboptimal route through R1 to reach 172.16.2.0.
The diagram is based on the network topology described in 9.2.1 diagram 1.
Using the no auto-summary command on each of the three routers changes the EIGRP behavior. This allows R1 and R2 to send individual routes for each of the 172.16.0.0/16 subnets.
The diagram depicts EIGRP automatically summarizing at major network boundaries using the default auto-summary command. This is shown in the routing table for R3, where a single 172.16.0.0/16 classful route is found.
D 172.16.0.0/16 [90/2172416] via 192.168.10.5, 01:08:30, Serial0/0/0
R3 uses this suboptimal route through R1 to reach 172.16.2.0.
The diagram is based on the network topology described in 9.2.1 diagram 1.
Using the no auto-summary command on each of the three routers changes the EIGRP behavior. This allows R1 and R2 to send individual routes for each of the 172.16.0.0/16 subnets.
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Because routes are no longer automatically summarized at major network boundaries, the EIGRP routing and topology tables also change.
Click R1, R2, and R3 in the figure.
Without automatic summarization, R3's routing table now includes the three subnets, 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24. Why does R3's routing table now have two equal cost paths to 172.16.3.0/24? Shouldn't the best path only be through R1 with the 1544 Mbps link?
Remember that EIGRP only uses the link with the slowest bandwidth when calculating the composite metric. The slowest link is the 64 Kbps link that contains the 172.16.3.0/30 network. In this example, the 1544 Mbps link and the 1024 Kbps link are irrelevant in the calculation as far as the bandwidth metric is concerned. Because both paths have the same number and types of outgoing interfaces, the delay values end up being the same. As a result, the EIGRP metric for both paths is the same, even though the path through R1 would actually be the "faster" path.
9.5.2 - Disabling Automatic Summarization
The diagram depicts that the EIGRP routing and topology tables change because routes are no longer automatically summarized at major network boundaries. Results are seen on each route by using the show i p e i g r p topology command.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The diagram depicts that the EIGRP routing and topology tables change because routes are no longer automatically summarized at major network boundaries. Results are seen on each route by using the show i p e i g r p topology command.
The diagram is based on the network topology described in 9.2.1 diagram 1.
9.5.3 Manual Summarization
Page 1:
EIGRP can be configured to summarize routes, whether or not automatic summarization (auto-summary) is enabled. Because EIGRP is a classless routing protocol and includes the subnet mask in the routing updates, manual summarization can include supernet routes. Remember, a supernet is an aggregation of multiple major classful network addresses.
Click R3 New LANs in the figure.
Suppose we added two more networks to router R3 using loopback interfaces: 192.168.2.0/24 and 192.168.3.0/24. We also configure networks in R3's EIGRP routing process with network commands so that R3 will propagate these networks to other routers.
Click Routing Tables 1 in the figure.
To verify that R3 sent EIGRP update packets to R1 and R2, we check the routing tables. In the figure, only the pertinent routes are shown. R1 and R2 routing tables show these additional networks in their routing tables: 192.168.2.0/24 and 192.168.3.0/24. Instead of sending three separate networks, R3 can summarize the 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24 networks as a single route.
Click R3 Summary Route in the figure.
Determining the Summary EIGRP Route
First, let's determine what the summary of these three networks would be using the same method we used to determine summary static routes:
1. Write out the networks that you want to summarize in binary.
2. To find the subnet mask for summarization, start with the left-most bit.
3. Work your way to the right, finding all the bits that match consecutively.
4. When you find a column of bits that do not match, stop. You are at the summary boundary.
5. Now, count the number of left-most matching bits, which in our example is 22. This number becomes your subnet mask for the summarized route: /22 or 255.255.252.0
6. To find the network address for summarization, copy the matching 22 bits and add all 0 bits to the end to make 32 bits.
The result is the summary network address and mask for 192.168.0.0/22.
Configure EIGRP Manual Summarization
To establish EIGRP manual summarization on all interfaces that send EIGRP packets, use the following interface command:
Router(config-if)#ip summary-address eigrp as-number network-address subnet-mask
Because R3 has two EIGRP neighbors, the EIGRP manual summarization in configured on both Serial 0/0/0 and Serial 0/0/1.
Click Routing Tables 2 in the figure.
The routing tables of R1 and R2 now no longer include the individual 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24 networks. Instead, they show a single summary route of 192.168.0.0/22. As you learned in Chapter 2, "Static Routing," summary routes lessen the number of total routes in routing tables, which makes the routing table lookup process more efficient. Summary routes also require less bandwidth utilization for the routing updates because a single route can be sent instead of multiple individual routes.
9.5.3 - Manual Summarization
The diagram depicts using the i p summary-address e i g r p command on R3 to manually summarize loopback class C addresses that have been added. The diagram is based on the network topology described in 9.2.1 diagram 1.
R3's New LAN's:
Two simulated LAN's are added to R3 using loopback interfaces. The networks are then configured as part of the EIGRP process. The show i p route command output on R1 and R2 shows three 192.168.x.x LAN's. By summarizing, the three networks can be advertised as one.
R3 Summary Route:
11000000 . 10101000 . 00000001
11000000 . 10101000 . 00000010
11000000 . 10101000 . 00000011
Configure the summary on each interface that sends EIGRP packets using the i p summary-address e i g r p 1 192.168.0.0 255.255.252.0 command.
The R1 routing table now contains a single summary route instead of three routes.
D 192.168.0.0/22 [90/2172416] via 192.168.10.6, 00:01:11, Serial0/0/1
The R2 routing table now contains a single summary route instead of three routes.
D 192.168.0.0/22 [90/3014400] via 192.168.10.10, 00:00:23, Serial0/0/1
The diagram depicts using the i p summary-address e i g r p command on R3 to manually summarize loopback class C addresses that have been added. The diagram is based on the network topology described in 9.2.1 diagram 1.
R3's New LAN's:
Two simulated LAN's are added to R3 using loopback interfaces. The networks are then configured as part of the EIGRP process. The show i p route command output on R1 and R2 shows three 192.168.x.x LAN's. By summarizing, the three networks can be advertised as one.
R3 Summary Route:
11000000 . 10101000 . 00000001
11000000 . 10101000 . 00000010
11000000 . 10101000 . 00000011
Configure the summary on each interface that sends EIGRP packets using the i p summary-address e i g r p 1 192.168.0.0 255.255.252.0 command.
The R1 routing table now contains a single summary route instead of three routes.
D 192.168.0.0/22 [90/2172416] via 192.168.10.6, 00:01:11, Serial0/0/1
The R2 routing table now contains a single summary route instead of three routes.
D 192.168.0.0/22 [90/3014400] via 192.168.10.10, 00:00:23, Serial0/0/1
9.5.4 EIGRP Default Route
Page 1:
Click R2 Static Default Configuration in the figure.
Using a static route to 0.0.0.0/0 as a default route is not routing protocol dependent. The "quad zero" static default route can be used with any currently supported routing protocols. The static default route is usually configured on the router that has a connection to a network outside the EIGRP routing domain, for example, to an ISP.
EIGRP requires the use of the redistribute static command to include this static default route with its EIGRP routing updates. The redistribute static command tells EIGRP to include this static route in its EIGRP updates to other routers. The figure shows the configuration of the static default route and the redistribute static command on router R2.
Note: The static default route is using the exit interface of Loopback1. This is because the ISP router in our topology does not physically exist. By using a loopback interface we can simulate a connection to another router.
Click R1, R2, and R3 in the figure.
The routing tables now show a static default route, and a gateway of last resort is now set.
In the routing tables for R1 and R3, notice the routing source and administrative distance for the new static default route. The entry for the static default route on R1 is the following:
D*EX 0.0.0.0/0 [170/3651840] via 192.168.10.6, 00:01:08, Serial0/1
- D - This static route was learned from an EIGRP routing update.
- * - The route is a candidate for a default route.
- EX - The route is an external EIGRP route, in this case a static route outside of the EIGRP routing domain.
- 170 - This is the administrative distance of an external EIGRP route.
Note: There is another method to propagate a default route in EIGRP, using the ip default-network command. More information on this command can be found at:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml
9.5.4 - EIGRP Default Route
The diagram depicts using a static route to 0.0.0.0/0 (quad zero) as a default route. The static default route is configured on the router that has a connection to a network outside the EIGRP routing domain, for example, to an ISP.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are used on R2 to configure the static default route.
R2(config)#i p route 0.0.0.0 0.0.0.0 loopback 1
R2(config)#router e i g r p 1
R2(config-router)#redistribute static
The R1 output for the show i p route command now contains an EIGRP external route (D*EX) to the default route.
D*EX 0.0.0.0/0 [170/3651840] via 192.168.10.6, 00:02:14, Serial0/0/1
The R3 output for the show i p router command now contains an EIGRP external route (D*EX) to the default route.
D*EX 0.0.0.0/0 [170/3139840] via 192.168.10.9, 00:01:25, Serial0/0/1
The diagram depicts using a static route to 0.0.0.0/0 (quad zero) as a default route. The static default route is configured on the router that has a connection to a network outside the EIGRP routing domain, for example, to an ISP.
The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are used on R2 to configure the static default route.
R2(config)#i p route 0.0.0.0 0.0.0.0 loopback 1
R2(config)#router e i g r p 1
R2(config-router)#redistribute static
The R1 output for the show i p route command now contains an EIGRP external route (D*EX) to the default route.
D*EX 0.0.0.0/0 [170/3651840] via 192.168.10.6, 00:02:14, Serial0/0/1
The R3 output for the show i p router command now contains an EIGRP external route (D*EX) to the default route.
D*EX 0.0.0.0/0 [170/3139840] via 192.168.10.9, 00:01:25, Serial0/0/1
9.5.5 Fine-tuning EIGRP
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The last two topics of this chapter discuss two fundamental ways to fine-tune EIGRP operations. First, we will discuss EIGRP bandwidth utilization. Next, we will discuss how to change the default hello and hold time values.
EIGRP Bandwidth Utilization
By default, EIGRP will use only up to 50 percent of the bandwidth of an interface for EIGRP information. This prevents the EIGRP process from over-utilizing a link and not allowing enough bandwidth for the routing of normal traffic. The ip bandwidth-percent eigrp command can be used to configure the percentage of bandwidth that may be used by EIGRP on an interface.
Router(config-if)#ip bandwidth-percent eigrp as-number percent
In the figure, R1 and R2 share a very slow 64kbps link. The configuration to limit how much bandwidth EIGRP uses is shown, along with the bandwidth command. The ip bandwidth-percent eigrp command uses the amount of configured bandwidth (or the default bandwidth) when calculating the percent that EIGRP can use. In our example, we are limiting EIGRP to no more than 50 percent of the link's bandwidth. Therefore, EIGRP will never use more the 32kbps of the link's bandwidth for EIGRP packet traffic.
9.5.5 - Fine-tuning EIGRP
The diagram depicts tuning EIGRP bandwidth utilization. The diagram is based on the network topology described in 9.2.1 diagram 1, with the addition of the loopback 2 and loopback 3 interfaces on R3.
The following commands are used on R1 and R2 to configure the bandwidth utilization on an interface and limit EIGRP to no more than 50% use of it.
R1(config)#interface serial 0/0/0
R1(config-i f)#bandwidth 64
R1(config-i f)#i p bandwidth-percent e i g r p 1 50
R2(config)#interface serial 0/0/0
R2(config-i f)#bandwidth 64
R2(config-i f)#i p bandwidth-percent e i g r p 1 50
The diagram depicts tuning EIGRP bandwidth utilization. The diagram is based on the network topology described in 9.2.1 diagram 1, with the addition of the loopback 2 and loopback 3 interfaces on R3.
The following commands are used on R1 and R2 to configure the bandwidth utilization on an interface and limit EIGRP to no more than 50% use of it.
R1(config)#interface serial 0/0/0
R1(config-i f)#bandwidth 64
R1(config-i f)#i p bandwidth-percent e i g r p 1 50
R2(config)#interface serial 0/0/0
R2(config-i f)#bandwidth 64
R2(config-i f)#i p bandwidth-percent e i g r p 1 50
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Configuring Hello Intervals and Hold Times
Hello intervals and hold times are configurable on a per-interface basis and do not have to match with other EIGRP routers to establish adjacencies. The command to configure a different hello interval is:
Router(config-if)#ip hello-interval eigrp as-number seconds
If you change the hello interval, make sure that you also change the hold time to a value equal to or greater than the hello interval. Otherwise, neighbor adjacency will go down after the hold time expires and before the next hello interval. The command to configure a different hold time is:
Router(config-if)#ip hold-time eigrp as-number seconds
The seconds value for both hello and hold time intervals can range from 1 to 65,535. This range means that you can set the hello interval to a value of just over 18 hours, which may be appropriate for a very expensive dialup link. However, in the figure we configure both R1 and R2 to use a 60-second hello interval and 180-second hold time. The no form can be used on both of these commands to restore the default values.
9.5.5 - Fine-tuning EIGRP
The diagram depicts configuring hello intervals and hold times. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are used on R1 and R2 to configure hello intervals and hold times.
R1(config)#int s0/0/0
R1(config-i f)#i p hello-interval e i g r p 1 60
R1(config-i f)#i p hold-time e i g r p 1 180
R1(config-i f)#end
R2(config)#int s0/0/0
R2(config-i f)#i p hello-interval e i g r p 1 60
R2(config-i f)#i p hold-time e i g r p 1 180
R2(config-i f)#end
The diagram depicts configuring hello intervals and hold times. The diagram is based on the network topology described in 9.2.1 diagram 1.
The following commands are used on R1 and R2 to configure hello intervals and hold times.
R1(config)#int s0/0/0
R1(config-i f)#i p hello-interval e i g r p 1 60
R1(config-i f)#i p hold-time e i g r p 1 180
R1(config-i f)#end
R2(config)#int s0/0/0
R2(config-i f)#i p hello-interval e i g r p 1 60
R2(config-i f)#i p hold-time e i g r p 1 180
R2(config-i f)#end
9.6 EIGRP Configuration Labs
9.6.1 Basic EIGRP Configuration LabPage 1:
In this lab, you will learn how to configure the routing protocol EIGRP. A loopback address will be used on the R2 router to simulate a connection to an ISP, where all traffic that is not destined for the local network will be sent. Some segments of the network have been subnetted using VLSM. EIGRP is a classless routing protocol that can be used to provide subnet mask information in the routing updates. This will allow VLSM subnet information to be propagated throughout the network.
Click the lab icon for more details.
9.6.1 - Basic EIGRP Configuration Lab
Link to Hands-on Lab: Basic EIGRP Configuration Lab
In this lab, you learn how to configure the routing protocol EIGRP. A loopback address is used on the R2 router to simulate a connection to an ISP, where all traffic that is not destined for the local network is sent. Some segments of the network have been subnetted using VLSM. EIGRP is a classless routing protocol that can be used to provide subnet mask information in the routing updates. This allows VLSM subnet information to be propagated throughout the network.
Link to Hands-on Lab: Basic EIGRP Configuration Lab
In this lab, you learn how to configure the routing protocol EIGRP. A loopback address is used on the R2 router to simulate a connection to an ISP, where all traffic that is not destined for the local network is sent. Some segments of the network have been subnetted using VLSM. EIGRP is a classless routing protocol that can be used to provide subnet mask information in the routing updates. This allows VLSM subnet information to be propagated throughout the network.
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Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.1. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
9.6.1 - Basic EIGRP Configuration Lab
Link to Packet Tracer Exploration: Basic EIGRP Configuration Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.1.
Link to Packet Tracer Exploration: Basic EIGRP Configuration Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.1.
9.6.2 Challenge EIGRP Configuration Lab
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In this lab activity, you will be given a network address that must be subnetted using VLSM to complete the addressing of the network. A combination of EIGRP routing and static routing will be required so that hosts on networks that are not directly connected will be able to communicate with each other. EIGRP must be configured so that all IP traffic takes the shortest path to the destination address.
Click the lab icon for more details.
9.6.2 - Challenge EIGRP Configuration Lab
Link to Hands-on Lab: Challenge EIGRP Configuration Lab
In this lab activity, you are given a network address that must be subnetted using VLSM to complete the addressing of the network. A combination of EIGRP routing and static routing is required so that hosts on networks that are not directly connected can communicate with each other. EIGRP must be configured so that all IP traffic takes the shortest path to the destination address.
Link to Hands-on Lab: Challenge EIGRP Configuration Lab
In this lab activity, you are given a network address that must be subnetted using VLSM to complete the addressing of the network. A combination of EIGRP routing and static routing is required so that hosts on networks that are not directly connected can communicate with each other. EIGRP must be configured so that all IP traffic takes the shortest path to the destination address.
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Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.2. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
9.6.2 - Challenge EIGRP Configuration Lab
Link to Packet Tracer Exploration: Basic EIGRP Configuration Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.2.
Link to Packet Tracer Exploration: Basic EIGRP Configuration Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.2.
9.6.3 Troubleshooting EIGRP Configuration Lab
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In this lab, you will begin by loading corrupted configuration scripts on each of the routers. These scripts contain errors that will prevent end-to-end communication across the network. You will need to troubleshoot each router to determine the configuration errors, and then use the appropriate commands to correct the configurations. When you have corrected all of the configuration errors, all of the hosts on the network should be able to communicate with each other.
Click the lab icon for more details.
9.6.3 - Troubleshooting EIGRP Configuration Lab
Link to Hands-on Lab: Challenge EIGRP Troubleshooting Lab
In this lab, you begin by loading corrupted configuration scripts on each of the routers. These scripts contain errors that prevent end-to-end communication across the network. You need to troubleshoot each router to determine the configuration errors, and then use the appropriate commands to correct the configurations. When you have corrected all the configuration errors, all hosts on the network should be able to communicate with each other.
Link to Hands-on Lab: Challenge EIGRP Troubleshooting Lab
In this lab, you begin by loading corrupted configuration scripts on each of the routers. These scripts contain errors that prevent end-to-end communication across the network. You need to troubleshoot each router to determine the configuration errors, and then use the appropriate commands to correct the configurations. When you have corrected all the configuration errors, all hosts on the network should be able to communicate with each other.
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Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.3. Remember, however, that Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Use the Lab PDF for more details.
Click the Packet Tracer icon for more details.
9.6.3 - Troubleshooting EIGRP Configuration Lab
Link to Packet Tracer Exploration: EIGRP Troubleshooting Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.3.
Link to Packet Tracer Exploration: EIGRP Troubleshooting Lab
Use this Packet Tracer Activity to repeat a simulation of Lab 9.6.3.
9.7 Chapter Summary
9.7.1 Summary and ReviewPage 1:
Summary
EIGRP (Enhanced Interior Gateway Routing Protocol) is a classless, distance vector routing protocol released in 1992 by Cisco Systems. EIGRP is a Cisco proprietary routing protocol and an enhancement of another Cisco propriety protocol IGRP (Interior Gateway Routing Protocol). IGRP is a classful, distance vector routing protocol which is no longer supported by Cisco. EIGRP uses the source code of "D" for DUAL in the routing table. EIGRP has a default administrative distance of 90 for internal routes and 170 for routes imported from an external source, such as default routes.
EIGRP used PDMs (Protocol Dependent Modules) giving it the capability to support different Layer 3 protocols including IP, IPX and AppleTalk. EIGRP uses RTP (Reliable Transport Protocol) as the Transport layer protocol for the delivery of EIGRP packets. EIGRP uses reliable delivery for EIGRP updates, queries and replies; and uses unreliable delivery for EIGRP hellos and acknowledgments. Reliable RTP means an EIGRP acknowledgment must be returned.
Before any EIGRP updates are sent, a router must first discover their neighbors. This is done with EIGRP hello packets. On most networks EIGRP sends hello packets every 5 seconds. On multipoint nonbroadcast multiaccess networks (NBMA) such as X.25, Frame Relay, and ATM interface with access links of T1 (1.544 Mbps) or slower, Hellos are sent every 60 seconds. The hold time is three times the hello, or 15 seconds on most networks and 180 seconds on low speed NBMA networks.
The hello and hold-down values do not need to match for two routers to become neighbors. The show ip eigrp neighbors command is used to view the neighbor table and verify that EIGRP has established an adjacency with its neighbors.
EIGRP does not send periodic updates like RIP. EIGRP sends partial or bounded updates, which includes only the route changes and only to those routers that are affected by the change. EIGRP composite metric uses bandwidth, delay, reliability and load to determine best path. By default only bandwidth and delay are used. The default calculation is the slowest bandwidth plus the sum of the delays of the outgoing interfaces from the router to the destination network.
At the center of EIGRP is DUAL (Diffusing Update Algorithm). The DUAL finite state machine is used to determine best path and potential backup paths to every destination network. The successor is a neighboring router that is used to forward the packet using the least-cost route to the destination network. Feasible distance (FD) is the lowest calculated metric to reach the destination network through the successor. A feasible successor (FS) is a neighbor who has a loop-free backup path to the same network as the successor, and also meets the feasibility condition. The feasibility condition (FC) is met when a neighbor's reported distance (RD) to a network is less than the local router's feasible distance to the same destination network. The reported distance is simply an EIGRP neighbor's feasible distance to the destination network.
EIGRP is configured with the router eigrp autonomous-system command. The autonomous-system value is actually a process-id and must be the same on all routers in the EIGRP routing domain. The network command is similar to that used with RIP. The network is the classful network address of the directly connected interfaces on the router. A wildcard mask is an optional parameter that can be used to include only specific interfaces.
There are several ways to propagate a static default route with EIGRP. The redistribute static command in EIGRP router mode is a common method.
9.7.1 - Summary and Review
In this chapter, you learned to:
- Describe the background and history of EIGRP.
- Describe the features and operation of EIGRP.
- Examine the basic EIGRP configuration commands and identify their purposes.
- Calculate the composite metric used by EIGRP.
- Describe the concepts and operation of DUAL.
- Describe the uses of additional configuration commands in EIGRP.
In this chapter, you learned to:
- Describe the background and history of EIGRP.
- Describe the features and operation of EIGRP.
- Examine the basic EIGRP configuration commands and identify their purposes.
- Calculate the composite metric used by EIGRP.
- Describe the concepts and operation of DUAL.
- Describe the uses of additional configuration commands in EIGRP.
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9.7.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. What is the routing algorithm used by EIGRP?
Answer: DUAL (Diffusing Update Algorithm)
Question 2. Does EIGRP send periodic updates?
Answer: No, EIGRP sends bounded updates, with only the routing information that is needed and only to those routers that need it.
Question 3. What command can be used to verify that EIGRP has established relationships with its directly connected neighbors?
Answer: show i p e i g r p neighbors
Question 4. What metrics does the EIGRP composite metric use? Which ones are used by default?
Answer: Bandwidth, delay, reliability, and load. Only bandwidth and delay are used by default.
Question 5. What is the feasibility condition?
Answer: When the neighbor's reported distance (RD) to a network is less than the router's feasible distance to the same destination network.
Question 6. Does EIGRP use automatic summarization similar to RIP? If so, how can it be disabled?
Answer: Yes. Automatic summarization can be disabled using the no auto-summary command.
Question 7. When enabling EIGRP with the router-mode command, router e i g r p autonomous-system, what are the requirements regarding the autonomous-system parameter?
Answer: The autonomous system needs to be the same on all routers in the EIGRP routing domain. The autonomous system number does not need to be an actual A S number.
Question 8. Below is output from the show i p e i g r p topology command. Does this output show a feasible successor? How can you tell?
P 192.168.10.4/30, 1 successors, FD is 3523840
via 192.168.10.10 (3523840/2169856), Serial0/1
via 172.16.3.1 (41024000/216856), Serial0/0
Answer: Yes, 172.16.3.1 is a feasible successor. This is because the RD of 2169856 is less than the successor 192.168.10.10 FD of 3523840.
Question 9. Why is a null0 summary route sometimes automatically included by EIGRP?
Answer: A null0 summary route is automatically added to the routing table by EIGRP when there is at least one child route with a routing source of EIGRP and the default auto-summary command is used.
This is a review and is not a quiz. Questions and answers are provided.
Question 1. What is the routing algorithm used by EIGRP?
Answer: DUAL (Diffusing Update Algorithm)
Question 2. Does EIGRP send periodic updates?
Answer: No, EIGRP sends bounded updates, with only the routing information that is needed and only to those routers that need it.
Question 3. What command can be used to verify that EIGRP has established relationships with its directly connected neighbors?
Answer: show i p e i g r p neighbors
Question 4. What metrics does the EIGRP composite metric use? Which ones are used by default?
Answer: Bandwidth, delay, reliability, and load. Only bandwidth and delay are used by default.
Question 5. What is the feasibility condition?
Answer: When the neighbor's reported distance (RD) to a network is less than the router's feasible distance to the same destination network.
Question 6. Does EIGRP use automatic summarization similar to RIP? If so, how can it be disabled?
Answer: Yes. Automatic summarization can be disabled using the no auto-summary command.
Question 7. When enabling EIGRP with the router-mode command, router e i g r p autonomous-system, what are the requirements regarding the autonomous-system parameter?
Answer: The autonomous system needs to be the same on all routers in the EIGRP routing domain. The autonomous system number does not need to be an actual A S number.
Question 8. Below is output from the show i p e i g r p topology command. Does this output show a feasible successor? How can you tell?
P 192.168.10.4/30, 1 successors, FD is 3523840
via 192.168.10.10 (3523840/2169856), Serial0/1
via 172.16.3.1 (41024000/216856), Serial0/0
Answer: Yes, 172.16.3.1 is a feasible successor. This is because the RD of 2169856 is less than the successor 192.168.10.10 FD of 3523840.
Question 9. Why is a null0 summary route sometimes automatically included by EIGRP?
Answer: A null0 summary route is automatically added to the routing table by EIGRP when there is at least one child route with a routing source of EIGRP and the default auto-summary command is used.
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The Packet Tracer Skills Integration Challenge Activity integrates all the knowledge and skills you acquired in previous chapters of this course and prior courses. Skills related to this chapter's discussion of EIGRP are also included.
In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you must implement a network design that satisfies the specifications. Next, you implement an effective EIGRP routing configuration with integrated default routing. Detailed instructions are provided within the activity.
Packet Tracer Skills Integration Instructions (PDF)
Click the Packet Tracer icon for more details.
9.7.1 - Summary and Review
Link to Packet Tracer Exploration: Packet Tracer Skills Integration Challenge
The Packet Tracer Skills Integration Challenge Activity integrates all the knowledge and skills you acquired in previous chapters of this course and prior courses. Skills related to this chapter's discussion of EIGRP are also included.
In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you implement a network design that satisfies the specifications. Next, you implement an effective EIGRP routing configuration with integrated default routing. Detailed instructions are provided within the activity.
Link to Packet Tracer Exploration: Packet Tracer Skills Integration Challenge
The Packet Tracer Skills Integration Challenge Activity integrates all the knowledge and skills you acquired in previous chapters of this course and prior courses. Skills related to this chapter's discussion of EIGRP are also included.
In this activity, you build a network from the ground up. Starting with an addressing space and network requirements, you implement a network design that satisfies the specifications. Next, you implement an effective EIGRP routing configuration with integrated default routing. Detailed instructions are provided within the activity.
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To Learn More
Routing TCP/IP, Volume I
There are several good sources to learn more about DUAL. Routing TCP/IP, Volume I by Jeff Doyle and Jennifer Carroll, includes an excellent section on the Diffusing Update Algorithm including two diffusing computation examples.
J.J. Garcia-Luna-Aceves
DUAL was first proposed by E.W. Dijkstra and C.S. Scholten, with the most prominent work done by that of J.J. Garcia-Luna-Aceves. J.J. Garcia-Luna-Aceves is the Jack Baskin Chair of Computer Engineering at the University of California, Santa Cruz (UCSC), and is a Principal Scientist at the Palo Alto Research Center (PARC). Several of J.J. Garcia-Luna-Aceves's published articles including his work done on DUAL, "Loop-Free Routing Using Diffusing Computations", IEEE/ACM Transactions on Networking, Vol. 1, No. 1, February 1993, can be found at: http://ccrg.soe.ucsc.edu/publications.html.
9.7.1 - Summary and Review
The diagram depicts a collage of people using computers and networks.
The diagram depicts a collage of people using computers and networks.
9.8 Chapter Quiz
9.8.1 Chapter QuizPage 1:
9.8.1 - Chapter Quiz
1. What is the purpose of the EIGRP PDM?
A. PDM is the Layer 4 protocol that EIGRP uses to share routing information.
B. PDM is the mechanism that EIGRP uses to ensure the availability of neighboring routers.
C. PDM is the algorithm engine used by EIGRP to create routing tables.
D. PDM provides modular support for Layer 3 protocols.
E. PDM is the distance to a destination as reported by a neighboring router.
2. Match the EIGRP terms and concepts with the correct descriptions. Not all options are used.
Options:
1. neighbor table
2. topology table
3. routing table
4. successor route
5. feasible successor route
6. summary route
Descriptions:
A. contains the EIGRP routes to be used for packet forwarding
B. primary route to be used, selected by DUAL
C. most important EIGRP data source; lists adjacent routers
D. backup path to a destination network
E. contains all learned routes to all destination networks
3. What type of EIGRP packet is used to discover, verify, and rediscover neighboring routers?
A. acknowledgement
B. hello
C. query
D. reply
4. If an EIGRP route goes down and a feasible successor is not found in the topology table, how does DUAL flag the route that has failed?
A. recomputed
B. passive
C. active
D. down
E. unreachable
F. successor
5. Which three tables does a router running EIGRP maintain? (Choose three.)
A. DUAL
B. feasible distance
C. neighbor
D. OSPF
E. routing
F. topology
6. What is the purpose of the EIGRP neighbor and topology tables?
A. DUAL uses the neighbor and topology tables to build the routing table.
B. The neighbor table is sent to all neighboring routers, which use it to build topology tables.
C. The neighbor table is sent to all routers listed in the neighbor table.
D. DUAL uses the neighbor table to create the topology table.
E. The neighbor table is broadcast to neighbor routers, while the topology table is broadcast to all other routers.
7. Refer to the command output described below. What does the 255/255 value in the output represent?
A# show interface Serial0/0
Serial0/0 is up, line protocol is up (connected)
Hardware is HD64570
Internet address is 10/1/1/1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
output omitted
Answers:
A. number of times that the link was in operation during 255 polls
B. link failure rate over 255 seconds
C. probability that the link will continue to be operational
D. static value representing the normal reliability of an interface type
8. Match the DUAL term with the correct definition.
Terms:
1. Feasible successor
2. Successor
3. Reported distance
4. Feasible distance
5. Routing table
6. Topology table
Definition:
A. visible backup path to a network
B. route that is used for packet forwarding and is the least-cost route
C. lowest calculated metric to reach the destination network
D. table that contains successors and feasible successors
E. table that contains successors only
9. A network administrator is troubleshooting an EIGRP routing issue. What command shows the administrator all possible paths to destination?
A. show i p route
B. show i p e i g r p topology active
C. show i p e i g r p neighbors detail
D. show i p e i g r p topology all-links
E. show i p e i g r p topology summary
10. Refer to the command described output below. What reported distance is the feasible successor to network 192.168.1.0 advertising?
Router1# show ip e i g r p topology
output omitted
P 192.168.10.0/24, 1 successors, FD is 3011840
via Summary (3011840/0), Null0
via Summary (41024000/2169856), Serial0/0/0
P 192.168.10.4/30, 1 successors, FD is 3523840
via 192.168.10.10 (3523840/2169856), Serial0/0/1
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
Answers:
A. 28160
B. 3014400
C. 2172416
D. 41026560
1. What is the purpose of the EIGRP PDM?
A. PDM is the Layer 4 protocol that EIGRP uses to share routing information.
B. PDM is the mechanism that EIGRP uses to ensure the availability of neighboring routers.
C. PDM is the algorithm engine used by EIGRP to create routing tables.
D. PDM provides modular support for Layer 3 protocols.
E. PDM is the distance to a destination as reported by a neighboring router.
2. Match the EIGRP terms and concepts with the correct descriptions. Not all options are used.
Options:
1. neighbor table
2. topology table
3. routing table
4. successor route
5. feasible successor route
6. summary route
Descriptions:
A. contains the EIGRP routes to be used for packet forwarding
B. primary route to be used, selected by DUAL
C. most important EIGRP data source; lists adjacent routers
D. backup path to a destination network
E. contains all learned routes to all destination networks
3. What type of EIGRP packet is used to discover, verify, and rediscover neighboring routers?
A. acknowledgement
B. hello
C. query
D. reply
4. If an EIGRP route goes down and a feasible successor is not found in the topology table, how does DUAL flag the route that has failed?
A. recomputed
B. passive
C. active
D. down
E. unreachable
F. successor
5. Which three tables does a router running EIGRP maintain? (Choose three.)
A. DUAL
B. feasible distance
C. neighbor
D. OSPF
E. routing
F. topology
6. What is the purpose of the EIGRP neighbor and topology tables?
A. DUAL uses the neighbor and topology tables to build the routing table.
B. The neighbor table is sent to all neighboring routers, which use it to build topology tables.
C. The neighbor table is sent to all routers listed in the neighbor table.
D. DUAL uses the neighbor table to create the topology table.
E. The neighbor table is broadcast to neighbor routers, while the topology table is broadcast to all other routers.
7. Refer to the command output described below. What does the 255/255 value in the output represent?
A# show interface Serial0/0
Serial0/0 is up, line protocol is up (connected)
Hardware is HD64570
Internet address is 10/1/1/1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
output omitted
Answers:
A. number of times that the link was in operation during 255 polls
B. link failure rate over 255 seconds
C. probability that the link will continue to be operational
D. static value representing the normal reliability of an interface type
8. Match the DUAL term with the correct definition.
Terms:
1. Feasible successor
2. Successor
3. Reported distance
4. Feasible distance
5. Routing table
6. Topology table
Definition:
A. visible backup path to a network
B. route that is used for packet forwarding and is the least-cost route
C. lowest calculated metric to reach the destination network
D. table that contains successors and feasible successors
E. table that contains successors only
9. A network administrator is troubleshooting an EIGRP routing issue. What command shows the administrator all possible paths to destination?
A. show i p route
B. show i p e i g r p topology active
C. show i p e i g r p neighbors detail
D. show i p e i g r p topology all-links
E. show i p e i g r p topology summary
10. Refer to the command described output below. What reported distance is the feasible successor to network 192.168.1.0 advertising?
Router1# show ip e i g r p topology
output omitted
P 192.168.10.0/24, 1 successors, FD is 3011840
via Summary (3011840/0), Null0
via Summary (41024000/2169856), Serial0/0/0
P 192.168.10.4/30, 1 successors, FD is 3523840
via 192.168.10.10 (3523840/2169856), Serial0/0/1
P 192.168.1.0/24, 1 successors, FD is 3014400
via 192.168.10.10 (3014400/28160), Serial0/0/1
via 172.16.3.1 (41026560/2172416), Serial0/0/0
Answers:
A. 28160
B. 3014400
C. 2172416
D. 41026560
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