350-401 Infrastructure Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Authentication is configured on one router but not on the other.

  Why this is correct

  EIGRP authenticates each hello packet and every routing update using a configured key, typically MD5 or SHA-2. If one router has authentication enabled (e.g., 'ip authentication mode eigrp' and 'ip authentication key-chain eigrp') while the peer does not, the receiving router fails the authentication check and silently discards the hello, so no adjacency can ever form. Even a key mismatch or different key-chain name on both sides produces the same failure, making this a classic common cause of missing EIGRP neighbor relationships.

• ✗

  The network statement uses an incorrect subnet mask.

  Why it's wrong here

  When using a network statement in classic EIGRP, you specify a network and a wildcard mask, not a subnet mask — for example, 'network 10.10.10.0 0.0.0.255'. If an admin mistakenly types a subnet mask like 255.255.255.0, the wildcard is interpreted as '0.0.0.255'? Actually the IOS may reject it or treat it as an ACL wildcard, leading to the interface not being included in EIGRP. This could result in hellos not being sent on that segment, which prevents adjacency, but the question specifically lists 'incorrect subnet mask' as the wrong answer because the EIGRP network statement nomenclature deals with wildcard masks, not subnet masks. Even if the wildcard is wrong, it does not necessarily prevent adjacency on a directly connected interface if that interface's IP is still included by another network statement.

• ✗

  One router has a loopback interface that is not advertised.

  Why it's wrong here

  A loopback interface that is not advertised via an EIGRP network statement has no bearing on forming a neighbor adjacency because adjacencies are established on the interfaces used to physically or logically connect peers, not on the loopback. EIGRP does not require every interface to be in the routing process; it only needs the interface traversed by the neighbor packets to be covered by a network statement. Loopbacks often act as the router ID or are injected for management purposes, but their absence from EIGRP does not break hellos or neighbor relationships with directly connected routers.

• ✗

  The hello and hold timers do not match.

  Why it's wrong here

  EIGRP timers do not need to match for two routers to become adjacent, which is a key difference from OSPF. Each router independently sends hellos every 'hello-interval' and waits a 'hold-time' to declare a neighbor dead; the router receiving a hello learns the source's hello interval, but uses its own locally configured hold time as the timeout. If hold time is shorter than the remote hello interval, the neighbor will time out and flap, but mismatched timers alone will not prevent the initial adjacency from forming. The question correctly marks this as wrong because a mismatch alone is not a barrier to adjacency; it can cause instability only if the hold time is too short.

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