CCNP Route FAQ: EIGRP Overview and Neighbor Relationships

CCNP Route FAQ: EIGRP Overview and Neighbor Relationships

Q1. 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?
e. None of the other answers is correct.

Answer: B and C. The network command tells IOS to match the first three octets when comparing the interface IP addresses to the configured “” value. Only two answers match in the first three octets. The other two answers have a 0 in the 3 rd octet, making the addresses not match the network command.

Figure: EIGRP Discovery and Update Process

Q2. 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 the reason for the omission? (Choose two answers.)
a. R1 has EIGRP neighbors reachable via S0/0 and S0/1, but not via S0/2, so it is not included.
b. S0/2 may currently be in a state other than up/up.
c. The network command requires the use of mask due to EIGRP being classful by default.
d. S0/2 may be configured as a passive interface.

Answer: B. The show ip eigrp interfaces command displays working (up/up) interfaces on which EIGRP has been enabled but omits passive interfaces. A failure of the interface, or making the interface passive, would omit the interface from the output of this command.

Q3. 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?
a. The show ip eigrp neighbors command on R1 lists the revised Hello timer.
b. The show ip eigrp interfaces command on R1 lists the revised Hello timer.
c. The R1-R2 neighborship fails due to Hello timer mismatch.
d. The show ip eigrp interfaces detail command on R1 lists the revised Hello timer.

Answer: D. The show ip eigrp interfaces detail command does display a router’s EIGRP Hello timer setting for each enabled interface. The other listed commands do not display the timer. Also, EIGRP routers do not have to have matching Hello timers to become neighbors.

Q4. Routers R1 and R2, currently EIGRP neighbors over their Fa0/0 interfaces (respectively), both use EIGRP authentication. Tuesday at 8 p.m. the neighborship fails.
Which of the following would not be useful when investigating whether authentication had anything to do with the failure?
a. debug eigrp packet
b. show key chain
c. show ip eigrp neighbor failure
d. show clock

Answer: C. The show ip eigrp neighbors failure command is not a valid IOS command. The debug displays messages that state when a neighborship fails due to authentication each time Hellos are exchanged. The show key chain command lists the specific keys that are currently valid, allowing you to determine if both routers use the same key values in currently valid keys. The show clock command displays the current time-of-day clock setting on a router, allowing you to check the valid times of the various keys versus the two router’s clocks.

Q5. 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 via 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?
a. Fa0/0 fails.
b. The command is rejected.
c. The existing three neighbors fail.
d. The neighborship with fails and then reestablishes.
e. None of the other answers is correct.

Answer: C. The neighbor fa0/0 command would only be rejected if the IP address ( is not inside the range of addresses in the subnet (, range– This command does not impact interface state. The command does disable all EIGRP multicasts, and because the three dynamically discovered neighbors require the EIGRP multicasts, all three neighbors fail. Although is a valid potential neighbor, both routers must be configured with static neighbor commands, and we know that was not previously configured with a static neighbor command; otherwise, it could not have been a neighbor with R1.

Q6. Which of the following settings could prevent two potential EIGRP neighbors from becoming neighbors? (Choose two answers.)
a. The interface used by one router to connect to the other router is passive in the EIGRP process.
b. Duplicate EIGRP router IDs.
c. Mismatched Hold Timers.
d. IP addresses of and, respectively.

Answer: A and D. Table lists the issues. For EIGRP, Router IDs do not have to be unique for EIGRP routers to become neighbors, and the hold timer does not have to match between the two neighbors. However, making an interface passive disables the processing of all EIGRP messages on the interface, preventing all neighborships. Mismatched IP subnets also prevent neighborships from forming.

Q7. 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 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
a. The configuration is missing one authentication-related configuration command.
b. The configuration is missing two authentication-related configuration commands.
c. Authentication type 9 is not supported; type 5 should be used instead.
d. The key numbers must begin with key 1, so change the key 3 command to key 1.

Answer: A. The configuration requires the ip authentication mode eigrp asn md5 command, which is currently missing. This command enables MD5-style authentication, rather than the default of no authentication. Adding this one command completes the configuration. Any valid key numbers can be used. Also, the 9 in the ip authentication key-chain eigrp 9 fred command refers to the EIGRP ASN, not an authentication type.

Q8. 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?
a. A partial mesh totaling 100: one between the central-site router and each of the 100 branches.
b. A full mesh – (101 * 100) / 2 = 5050–One neighborship between each pair of routers.
c. 101–One between each router (including the central site) and its nearby PE router.
d. None of the answers is correct.

Answer: A. EIGRP forms neighborships only when two routers can communicate directly over a data link. As a result, with Frame Relay, EIGRP neighborships occur only between routers on the ends of a PVC, so in this case, 100 neighborships exist.

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James Palmer

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