350-401 Question #1035: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. Two indirectly connected routers fail to form an OSPF neighborship. What is the cause of the issue?

Options

• Afailing hello packets between the two routers
• BDR/BDR selection dispute
• CMTU mismatch
• DOSPF network type mismatch

Explanation

An OSPF MTU mismatch between directly connected interfaces is a common reason for OSPF neighborship failure, causing the adjacency to get stuck in the EXSTART or EXCHANGE state.

Common mistakes.

• A. Failing hello packets would prevent the neighborship from forming beyond the INIT state, which is an earlier stage than the EXSTART/EXCHANGE state often associated with MTU mismatches.
• B. DR/BDR selection dispute could cause issues or delays, but an MTU mismatch directly prevents the DBD exchange necessary for full adjacency, which is a more fundamental problem for two routers trying to become full neighbors.
• D. An OSPF network type mismatch (e.g., point-to-point vs. broadcast) can prevent neighborship formation, but an MTU mismatch is a specific and common cause that often results in the EXSTART/EXCHANGE state.

Concept tested. OSPF MTU mismatch issues

Reference. https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13682-10.html#mtu

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