Why Does OSPF Fail to Establish Adjacency Between R1 and R2?

350-401 By islever Editorial Last updated

Short answer

B interface MTU mismatch

Question

Refer to the exhibit. Why does OSPF fail to establish an adjacency between R1 and R2? - Exhibit: Why Does OSPF Fail to Establish Adjacency Between R1 and R2?
  1. authentication mismatch
  2. interface MTU mismatch Correct Answer
  3. timers mismatch
  4. area mismatch

Key Takeaways

This question tests your understanding of OSPF's database description (DBD) exchange phase, where a mismatch in interface MTU prevents adjacency from progressing beyond EXSTART state, a common trap for confusing with timer or area mismatches.

OSPF adjacency can fail due to interface MTU mismatch, leaving neighbors stuck in EXSTART state. Community consensus confirms option B as the correct answer for this exhibit-based question.

Common mistake: Choosing timers mismatch (option C) is a common wrong answer because OSPF hello/dead timer mismatches also prevent adjacency, but the exhibit's retransmission and DBD packet size indicators point to MTU mismatch, not timer issues.

Expert Analysis

Why the Answer Is Correct

OSPF neighbors must have matching MTU values on the interface where they are adjacent. During the DBD exchange, the MTU is included in the DBD packet. If the receiving router has a smaller MTU than the advertised one, it will reject the DBD packet and leave the neighbor in EXSTART state, causing retransmissions and no adjacency. The exhibit shows 'retransmissions' and the debug output confirms 'has smaller interface MTU', directly pointing to option B, interface MTU mismatch.

Why the Other Options Are Wrong

Authentication mismatch (A) would cause OSPF packets to be dropped or ignored, typically not allowing the state to even reach EXSTART. Timers mismatch (C) would prevent hello exchange and keep neighbors stuck in DOWN or INIT, not EXSTART. Area mismatch (D) would cause OSPF to ignore hello packets because the area ID does not match, again preventing neighbor state progression. The debug output clearly shows EXSTART and DBD-related errors, which are specific to MTU mismatches.

Community Comment Notes

One comment highlights 'MTU is the maximum packet size' and mentions 'retransmissions', which is a key clue in the exhibit. Another commenter recreated the lab and showed the exact log: 'Nbr 1.1.1.1 has smaller interface MTU', confirming MTU mismatch. A third comment mentions 'EXSTART state is the keyword', which is a strong diagnostic indicator for MTU or router ID mismatch—but since the exhibit focuses on MTU, B is correct.

Official Reference

https://www.rfc-editor.org/rfc/rfc2328

This link opens the official vendor documentation in a new tab.

Exam Strategy

When OSPF adjacency is stuck in EXSTART, always check interface MTU mismatch first—a common hidden trap. Use 'debug ip ospf adjacency' or 'show ip ospf interface' to verify MTU and quickly eliminate the other options by focusing on the state progression. Remember that timer, area, and authentication issues prevent adjacency before EXSTART, whereas MTU mismatch specifically blocks DBD exchange.

Community Votes

B
100%

100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 3 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 3

  1. chiacche 👍 3 Answered B
    keyword: retransmissions Maximum Transmission Unit (MTU) is the maximum packet size that each interface or link in the network can transmit. If the MTUs between two OSPF neighbors do not match, the neighbor relationship may not be established.
  2. NetworkJanitor 👍 2 Answered B
    It's B. Re-created in CML: Nov 20 22:11:01.285: OSPF-1 ADJ Gi1/0/1: First DBD and we are not SLAVE <snip> Nov 20 22:11:06.066: OSPF-1 ADJ Gi1/0/1: Rcv DBD from 1.1.1.1 seq 0x1172 opt 0x52 flag 0x7 len 32 mtu 1000 state EXSTART Nov 20 22:11:06.066: OSPF-1 ADJ Gi1/0/1: Nbr 1.1.1.1 has smaller interface MTU <snip> Nov 20 22:12:33.751: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet1/0/1 from EXSTART to DOWN, Neighbor Down: Too many retransmissions If you want to see what MTU value the neighbor is sending: debug ip ospf adj hth
  3. NetworkJanitor 👍 2 Answered B
    EXSTART state is the keyword (MTU or RID mismatch)

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