350-401 Architecture 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.

• ✗

  Dead timer mismatch

  Why it's wrong here

  An OSPF router declares a neighbor dead when no Hello is received within the dead interval, which is typically four times the hello interval. If timers differ, the peer with the shorter dead timer ages out the adjacency and brings it down, but the state machine resets back to DOWN and then attempts to re-establish, causing persistent flapping rather than a stable stuck-in-EXSTART condition. EXSTART is only reached after a two-way state, so timer mismatches manifest as repeated state resets and neighbor loss, not as an inability to transmit database description packets.

• ✗

  Authentication misconfiguration

  Why it's wrong here

  When OSPF authentication keys or types differ, an interface silently discards incoming Hello packets before they can be processed at the OSPF protocol level. Without a valid Hello from the peer, the neighbor is never accepted, so the adjacency remains in the INIT state (or the neighbor is not even listed) and cannot progress to EXSTART. This failure mode occurs earlier in the neighbor state machine, whereas an MTU problem only becomes apparent after two-way communication is established and database description packets are exchanged.

• ✗

  Mismatched OSPF area IDs

  Why it's wrong here

  A Hello packet carries the originating router's area ID, and a receiving router will only form an adjacency if that area ID matches one of its own configured OSPF areas. If the area IDs differ, the Hello is deemed invalid and is silently ignored, so the routers never reach even the two-way state that precedes EXSTART. Therefore, area ID mismatch prevents the neighbor relationship from ever being established, making it impossible to be stuck at the DB description exchange stage.

• ✓

  MTU mismatch between the routers

  Why this is correct

  During the EXSTART/EXCHANGE phase, OSPF routers exchange database description (DBD) packets that can be as large as the interface MTU; if one router's MTU is lower, the larger DBD packet will be dropped or fragmented, and the neighbor will never leave EXSTART because it keeps waiting for a valid DBD sequence. The interface MTU mismatch is a classic cause of OSPF adjacencies stuck in EXSTART, as the router with the smaller MTU silently discards the oversized multicast packets. This can be diagnosed by checking the 'show ip ospf neighbor' state and by ensuring both ends have the same MTU or by enabling 'ip ospf mtu-ignore' as a workaround.

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