Chapter 9: EIGRP

Cisco Press

Excerpt from CCIE Routing and Switching Official Exam Certification Guide, 2nd Edition.

Please note that the 2nd Edition of this book is no longer available. The most current edition is the 3rd edition

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Blueprint topics covered in this chapter:

This chapter covers the following topics from the Cisco CCIE Routing and Switching written exam blueprint:

IP Routing


The use of show and debug commands

This chapter covers most of the features, concepts, and commands related to EIGRP. Chapter 11, "IGP Route Redistribution, Route Summarization, and Default Routing," covers a few other details of EIGRP—in particular, route redistribution, route filtering when redistributing, and route summarization.

"Do I Know This Already?" Quiz

Table 9-1 outlines the major headings in this chapter and the corresponding "Do I Know This Already?" quiz questions.

Table 9-1: "Do I Know This Already" Foundation Topics Section-to-Question Mapping

Foundation Topics SectionQuestions Covered in This SectionScore
EIGRP Basics and Steady-State Operation1–4 
EIGRP Convergence5–7 
EIGRP Configuration8–9 
Total Score  

In order to best use this pre-chapter assessment, remember to score yourself strictly. You can find the answers in Appendix A, "Answers to the 'Do I Know This Already?' Quizzes."

  1. Which of the following items are true of EIGRP?

    1. Authentication can be done using MD5 or clear text.

    2. Uses UDP port 88.

    3. Sends full or partial updates as needed.

    4. Multicasts updates to

  2. Four routers (R1, R2, R3, and R4) are attached to the same VLAN. R1 has been configured for an EIGRP Hello timer of 3. R2 has been configured with a metric weights 0 0 0 1 0 0 command. R3 has been configured with a hold time of 11 seconds. Their IP addresses are,,, and, with /24 prefixes, except R4, which has a /23 prefix configured. All other related parameters are set to their default. Select the routers that are able to collectively form neighbor relationships.

    1. R1

    2. R2

    3. R3

    4. R4

    5. None of them can form a neighbor relationship.

  3. In the following command output, what do the numbers in the column labeled "H" represent?

    R1# show ip eigrp neighbors
    IP-EIGRP neighbors for process 1
    H Address         Interface    Hold Uptime      SRTT     RTO        Q         Seq
                                                   (sec)              (ms)        Cnt  Num
    2     Fa0/0           4 00:03:10       1     4500        0        233
    1   Fa0/0          11 00:04:43       1     4500        0         81
    0   Fa0/0          14 00:05:11    1927     5000        0         84
    1. The current Hold Time countdown

    2. The number of seconds before a Hello is expected

    3. The order in which the neighbors came up

    4. None of the other answers are correct

  4. Which of the following is not true regarding the EIGRP Update message?

    1. Updates require an acknowledgement with an Ack message.

    2. Updates can be sent to multicast address

    3. Updates are sent as unicasts when they are retransmitted.

    4. Updates always include all routes known by a router, with partial routing information distributed as part of the EIGRP Reply message.

  5. The output of a show ip eigrp topology command lists information about subnet, with two successors, and three routes listed on lines beginning with "via." How many feasible successor routes exist for

    1. 0

    2. 1

    3. 2

    4. 3

    5. Cannot determine from the information given

  6. The following command output shows R11's topology information for subnet Then R11 and R12 (IP address are connected to the same LAN segment. Then R11's EIGRP Hold Time expires for neighbor R12. Which of the following is true about R11's first reaction to the loss of its neighbor R12?

    R11# show ip eigrp topology
    ! lines omitted for brevity
    P, 1 successors, FD is 1456
     via (1456/1024), FastEthernet0/0
      via (1756/1424), Serial0/0.4
    1. R11 sends Updates to all neighbors poisoning its route to

    2. R11 replaces the old route through with the feasible successor route through

    3. R11 sends Query messages to all other neighbors to ensure that the alternate route through is loop free, before using the route.

    4. R11 first Queries only neighbors on interface fa0/0 for alternative routes before Querying the rest of its neighbors.

  7. EIGRP router R11 has just changed its route to subnet to the active state, and has sent a Query to five neighbors. Which of the following is true about the next step taken by R11?

    1. R11 adds a new route to to the routing table as soon as it receives an EIGRP Reply that describes a new route to

    2. R11 can add a new route to after receiving Reply messages from all 5 neighbors.

    3. R11 can add a new route for to the routing table, even without 5 Reply messages, once the Hold timer expires.

    4. R11 can add a new route for to the routing table, even without 5 Reply messages, once the Dead timer expires.

  8. EIGRP router R11 has five interfaces, with IP address (interface fa0/0),,,, and Its EIGRP configuration is shown below. Which of the following answers is true regarding this router?

    router eigrp 1
     passive-interface fa0/0
    1. R11 will send EIGRP Updates out fa0/0, but not process received EIGRP Updates.

    2. R11 will advertise connected subnets and

    3. R11 will advertise subnets and, as well as attempt to send Hellos and Updates on the related interfaces.

    4. The network command does not match any interfaces, so EIGRP will essentially do nothing.

  9. EIGRP router Br1 is a branch router with two Frame Relay subinterfaces (s0/0.1 and s0/0.2) connecting it to distribution routers. It also has one LAN interface, fa0/0. No other routers connect to the Br1 LAN. Which of the following scenarios prevent router Br1 from sending EIGRP Hellos out its fa0/0 interface?

    1. The inclusion of the passive-interface fa0/0 command on Br1

    2. The inclusion of the eigrp stub command on Br1

    3. The inclusion of the eigrp stub receive-only command on Br1

    4. The lack of a network command that matches the IP address of Br1's fa0/0 interface

Foundation Topics

EIGRP Basics and Steady-State Operation

Many CCIE candidates have known, at least at some point, many of the details of EIGRP operation and configuration. EIGRP is widely deployed and is thoroughly covered on the CCNP BSCI exam. With that in mind, this chapter strives to review the key terms and concepts briefly, and get right to specific examples that detail EIGRP operation on a Cisco router. To that end, the chapter begins with Table 9-2, which lists some of the key features related to EIGRP.

Table 9-2: EIGRP Feature Summary

Key PointFeatureDescription
 TransportIP, protocol type 88 (does not use UDP or TCP).
 MetricBased on constrained bandwidth and cumulative delay by default, and optionally load, reliability, and MTU.
 Hello intervalInterval at which a router sends EIGRP Hello messages on an interface.
 Hold timerTimer used to determine when a neighboring router has failed, based on a router not receiving any EIGRP messages, including Hellos, in this timer period.
 Update destination addressNormally sent to, with retransmissions being sent to each neighbor's unicast IP address.
 Full or partial updatesFull updates are used when new neighbors are discovered; otherwise, partial updates are used.
 AuthenticationSupports MD5 authentication only.
 VLSM/classlessEIGRP includes the mask with each route, also allowing it to support discontiguous networks and VLSM.
 Route TagsAllows EIGRP to tag routes as they are redistributed into EIGRP.
 Next-hop fieldSupports the advertisement of routes with a different next-hop router than the advertising router.
 Manual route summarizationAllows route summarization at any point in the EIGRP network.
 MultiprotocolSupports the advertisement of IPX and AppleTalk routes.

Hellos, Neighbors, and Adjacencies

After a router has been configured for EIGRP, and its interfaces come up, it attempts to find neighbors by sending EIGRP Hellos (destination Once a pair of routers have heard each other say Hello, they become adjacent—assuming several key conditions are met. Once neighbors pass the checks in the following list, they are considered to be adjacent. At that point, they can exchange routes and are listed in the output of the show ip eigrp neighbor command.

Must pass the authentication process

Must use the same configured AS number

Must believe that the source IP address of a received Hello is in that router's primary connected subnet on that interface

K values must match

The wording of the third item in the list bears a little further scrutiny. The primary subnet of an interface is the subnet as implied by the ip address command that does not have the secondary keyword. An EIGRP router looks at the source IP address of a Hello; if the source IP address is a part of that router's primary subnet of the incoming interface, the Hello passes the IP address check.

This logic leaves open some interesting possibilities. For example, if the routers are misconfigured with different subnet masks, the check may still pass. If one router has configured, and the other has configured, they could become adjacent, assuming all the other checks pass. While EIGRP supports secondary IP addresses and subnets, EIGRP sources its messages from the address in the primary subnet, and the IP addresses of neighbors must be in the subnet of the primary subnets.

The last item in the list mentions K values; K values are constants that define the multipliers used by EIGRP when calculating metrics. The settings can be changed with a router eigrp subcommand metric weights tos k1 k2 k3 k4 k5. The command defaults to a setting of 0 1 0 1 0 0, meaning that only bandwidth and delay are used to calculate the metric. (The examples in this chapter habitually change the settings to 0 0 0 1 0 0, which removes bandwidth from the calculation and makes the metrics in the examples a little more obvious.)

Besides simply checking to see if the right parameters agree, the Hello messages also serve as an EIGRP keepalive. Adjacent routers continue to multicast Hellos based on each interface's EIGRP hello interval. If a router fails to hear from a neighbor for a number of seconds, defined by the EIGRP Hold Time for that neighbor, all routes through the neighbor are considered to have failed.

Example 9-1 shows how a router displays some of the basic information regarding EIGRP operations based on Figure 9-1. The example begins with four routers (R1, R2, S1, and S2) that have only their common LAN interfaces up, just to show the Hello process. By the end of the example, the R2-to-R5 PVC will come up, but the EIGRP adjacency will fail due to a K-value mismatch.

Figure 9-1

Sample Internetwork Used for EIGRP Examples

Example 9-1: Forming EIGRP Adjacencies

! First, a debug is initiated on R1.
R1# debug eigrp packet hello
EIGRP Packets debugging is on
Jan 11 13:27:19.714: EIGRP: Received HELLO on FastEthernet0/0 nbr
Jan 11 13:27:19.714: AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ
  un/rely 0/0
! S2's LAN interface brought up, not shown
! Below, a pair of log messages appear, announcing the new neighbor; this message 
! appears due to the default router eigrp subcommand eigrp log-neighbor-changes.
Jan 11 13:27:19.995: EIGRP: New peer
Jan 11 13:27:19.995: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor   (FastEthernet0/0) is up: new adjacency
! Next, only neighbors who become adjacent—those that pass all the required 
! checks for the parameters—are listed. The Hold timer is shown; it starts at 
! its maximum, and decrements towards 0, being reset upon the receipt of any EIGRP
! packet from that neighbor. The "H" column on the left states the order in 
! which the neighbors became adjacent. 
R1# show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address     Interface  Hold Uptime SRTT RTO Q Seq
                                            (sec)         (ms)  Cnt Num
2    Fa0/0    4 00:03:10 1 4500 0 233
1   Fa0/0    11 00:04:43 1 4500 0 81
0   Fa0/0    14 00:05:11 1927 5000 0 84
! Below, the PVC between R2 and R5 came up, but R5's K values do not match R2's.
! Both messages below are log messages, with no debugs enabled on either router.
! Next message on R5 !!!!!!!!!!
03:55:51: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor (Serial0) is down: K-value   mismatch
! Next message on R2 !!!!!!!!!!
Jan 11 13:21:45.643: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor (Serial0/0.5)   is down: Interface Goodbye received

Note that when the PVC between R2 and R5 comes up, the message on R5 is pretty obvious, but the message at R2 says nothing about K values. Some later releases of Cisco IOS mistake invalid EIGRP K-value settings as a newer EIGRP message called a Goodbye message. Goodbye messages allow routers to tell each other that they are shutting down in a graceful fashion; be aware that this message may simply be the result of a K-value mismatch.

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