A township public-works department configures EIGRP authentication with a key chain on WAN interfaces so a rogue device plugged into a spare circuit cannot join the AS. What symptom typically appears when authentication fails between two otherwise valid neighbors?
Select an answer to reveal the explanation.
Short Explanation
Auth is the bouncer at the public-works door: without the right key-chain stamp, the other router never gets on the guest list. You may see hellos, but you will not get a solid neighbor—and that is exactly what blocks a rogue WAN plug.
Full Explanation
EIGRP authentication with a key chain on township public-works WAN interfaces validates neighbors before adjacency completes. When authentication fails—wrong key, missing key, lifetime not active—hellos may still be visible on the wire or in debugs, but the adjacency does not reach a stable neighbor state because the peer is rejected. That failure mode is intentional on spare circuits: a rogue device plugged into a public-works WAN drop cannot join the municipal AS and inject routes. Correct AS, K-values, and IP connectivity do not override failed authentication.
Claiming neighbors form successfully but only stub routes are accepted until the key chain is removed fails by concept: authentication failure blocks adjacency, not a selective stub-route filter after FULL/up state. Claiming the routing table fills with poison-reverse entries while authentication continues quietly invents a symptom unrelated to auth failure. Claiming OSPF automatically takes over the WAN because EIGRP auth failure triggers protocol migration fails—there is no such automatic IGP failover migration based on EIGRP key mismatch.
Exam caveat: platforms differ in MD5 versus key-chain/keyed HMAC styles and in exact debug counters; the exam-level symptom is adjacency failure, not a specific syslog string. Operational check: verify matching key-chain keys and overlapping accept/send lifetimes on both public-works peers, confirm the interface references the chain, and only then expect EIGRP neighbors to come up while unauthorized plugs still fail.