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.

• ✗

  The router ID is the same on both routers.

  Why it's wrong here

  A duplicate OSPF router ID causes a router to reject neighbor discovery packets that carry its own router ID, preventing the neighbor relationship from forming or causing it to flap. The adjacency typically cannot move beyond INIT or DOWN, and the error would not manifest as a neighbor stuck specifically in EXSTART state. Because the symptom here is an adjacency stuck in EXSTART, an RID conflict is a different failure pattern and is not the cause.

• ✗

  The area ID is different.

  Why it's wrong here

  An OSPF Hello packet carries the originating area ID; if the receiving interface belongs to a different area, the Hello is dropped as invalid and no neighbor adjacency ever forms. In this case, the routers would never progress past the INIT state or would remain in DOWN with the relevant 'area mismatch' debugs, rather than moving through 2WAY to EXSTART. A wrong area assignment therefore produces a much earlier failure in the OSPF neighbor state machine than the observed EXSTART hang.

• ✗

  The hello timer is set to 30 seconds on one router.

  Why it's wrong here

  When hello intervals differ, each router rejects the other's Hello packets because the Hello interval field does not match the local configuration, so the neighbor flips between DOWN and INIT. The routers never achieve the 2WAY state or proceed to database synchronization, meaning they cannot get stuck in EXSTART as seen in the scenario. The router may also show 'event timer' or 'dead timer' mismatches, but any timer mismatch keeps the adjacency in the earliest stages, not in EXSTART.

• ✓

  The interface MTU does not match.

  Why this is correct

  An MTU mismatch is the classic cause of an OSPF neighbor being stuck in EXSTART; during the Database Description exchange, each router advertises its outgoing interface MTU in the DBD packet header. If one router's interface has a lower MTU, it discards DBD packets that declare a larger MTU, so the neighboring router never receives a valid acknowledgment and remains in EXSTART. Because OSPF does not initialize the DBD exchange until both MTUs are verified equal, the adjacency stalls at the point where the Master/Slave election occurs, exactly matching the reported symptom.

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