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Fundamental EIGRP Concepts

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This chapter from CCNP Routing and Switching ROUTE 300-101 Official Cert Guide covers EIGRP fundamentals, EIGRP neighborships, and neighborships over WANs.
This chapter is from the book

Enhanced Interior Gateway Routing Protocol (EIGRP) is configured with a few relatively simple commands. In fact, for most any size network, you could go to every router, enter the router eigrp 1 command, followed by one or more network net-id subcommands (one for each classful network to which the router is connected), and EIGRP would likely work, and work very well, with no other configuration.

In spite of that apparent simplicity, here you sit beginning the first of four chapters of EIGRP coverage in this book. Many reasons exist for the amount of EIGRP material included here. First, EIGRP includes many optional configuration features that you need to both understand and master for the CCNP ROUTE exam. Many of these features require a solid understanding of EIGRP internals as well—a topic that can be conveniently ignored if you just do the minimal configuration, but something very important to planning, implementing, and optimizing a medium/large enterprise network.

Another reason for the depth of EIGRP coverage in this book is a fundamental change in the philosophy of the CCNP exams, as compared with earlier CCNP exam versions. Cisco has increased the focus on planning for the implementation and verification of new network designs. The bar has been raised, and in a way that is consistent with typical engineering jobs. Not only do you need to understand all the EIGRP features, but you also need to be able to look at a set of design requirements, and from that decide which EIGRP configuration settings could be useful—and which are not useful. You must also be able to direct others as to what verification steps would tell them if the implementation worked or not, rather than just relying on typing a ? and looking around for that little piece of information you know exists somewhere.

This chapter begins with the “EIGRP Fundamentals” section, which is a review of the core prerequisite facts about EIGRP. Following the review, the chapter examines EIGRP neighbor relationships, including a variety of configuration commands that impact neighbor relationships, and the verification commands that you can use to confirm how well EIGRP neighbors work.

“Do I Know This Already?” Quiz

The “Do I Know This Already?” quiz enables you to assess whether you should read the entire chapter. If you miss no more than one of these seven self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 4-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those headings so that you can assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A.

Table 4-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Foundation Topics Section


EIGRP Fundamentals

1, 2

EIGRP Neighborships


Neighborships over WANs


  1. A router has been configured with the commands router eigrp 9 and network No other EIGRP-related commands have been configured. The answers list the IP addresses that could be assigned to this router’s Fa0/0 interface. Which answers list an IP address/prefix length that would cause the router to enable EIGRP on Fa0/0? (Choose two answers.)

    5. None of the other answers are correct.
  2. Router R1 has working interfaces S0/0, S0/1, and S0/2, with IP address/prefix combinations of,, and R1’s configuration includes the commands router eigrp 9 and network The show ip eigrp interfaces command lists S0/0 and S0/1 in the command output, but not S0/2. Which answer gives a possible reason for the omission?

    1. R1 has EIGRP neighbors reachable through S0/0 and S0/1, but not through S0/2, so it is not included.
    2. S0/2 might currently be in a state other than up/up.
    3. The network command requires the use of mask because of EIGRP being classful by default.
    4. S0/2 might be configured as a passive interface.
  3. Routers R1 and R2 are EIGRP neighbors using their Fa0/0 interfaces, respectively. An engineer adds the ip hello-interval eigrp 9 6 command to R1’s Fa0/0 configuration. Which of the following is true regarding the results from this change?

    1. The show ip eigrp neighbors command on R1 lists the revised Hello timer.
    2. The show ip eigrp interfaces command on R1 lists the revised Hello timer.
    3. The R1-R2 neighborship fails because of a Hello timer mismatch.
    4. The show ip eigrp interfaces detail command on R1 lists the revised Hello timer.
  4. Router R1 has been configured with the commands router eigrp 9 and network, with no other current EIGRP configuration. R1’s (working) Fa0/0 interface has been configured with IP address R1 has found three EIGRP neighbors reachable through interface Fa0/0, including the router with IP address When the engineer attempts to add the neighbor fa0/0 command in EIGRP configuration mode, which of the following occurs?

    1. Fa0/0 fails.
    2. The command is rejected.
    3. The existing three neighbors fail.
    4. The neighborship with fails and then reestablishes.
    5. None of the other answers is correct.
  5. Which of the following settings could prevent two potential EIGRP neighbors from becoming neighbors? (Choose two answers.)

    1. The interface used by one router to connect to the other router is passive in the EIGRP process.
    2. Duplicate EIGRP router IDs.
    3. Mismatched Hold Timers.
    4. IP addresses of and, respectively.
  6. An engineer has added the following configuration snippet to an implementation planning document. The configuration will be added to Router R1, whose Fa0/0 interface connects to a LAN to which Routers R2 and R3 also connect. R2 and R3 are already EIGRP neighbors with each other. Assuming that the snippet shows all commands on R1 related to EIGRP authentication, which answer lists an appropriate comment to be made during the implementation plan peer review?

    key chain fred
    key 3
    key-string whehew
    interface fa0/0
    ip authentication key-chain eigrp 9 fred
    1. The configuration is missing one authentication-related configuration command.
    2. The configuration is missing two authentication-related configuration commands.
    3. Authentication type 9 is not supported; type 5 should be used instead.
    4. The key numbers must begin with key 1, so change the key 3 command to key 1.
  7. A company has a Frame Relay WAN with one central-site router and 100 branch office routers. A partial mesh of PVCs exists: one PVC between the central site and each of the 100 branch routers. Which of the following could be true about the number of EIGRP neighborships?

    1. A partial mesh totaling 100: one between the central-site router and each of the 100 branches.
    2. A full mesh — (101 * 100) / 2 = 5050 — One neighborship between each pair of routers.
    3. 101 — One between each router (including the central site) and its nearby PE router.
    4. None of the answers is correct.
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