Which two circumstances can prevent two routers from establishing an OSPF neighbor adjacency? (Choose two.)

Question

Options

• Amismatched autonomous system numbers
• Ban ACL blocking traffic from multicast address 224.0.0.10
• Cmismatched process IDs
• Dmismatched hello timers and dead timers
• Euse of the same router ID on both devices

• A
  4% (1)
• B
  4% (1)
• D
  92% (23)

Explanation

OSPF requires that neighboring routers agree on hello and dead timer intervals - a mismatch (D) causes routers to declare each other "down" before a relationship can form, blocking adjacency entirely. Duplicate router IDs (E) create an identity conflict that breaks the neighbor discovery process, since OSPF uses the router ID to uniquely identify each device in the topology.

Why the distractors are wrong:

• A - Autonomous system numbers belong to BGP, not OSPF; OSPF uses area IDs instead, and even mismatched areas don't always prevent adjacency (they just affect routing).
• B - 224.0.0.10 is the PIM multicast address, not OSPF. OSPF uses 224.0.0.5 (all OSPF routers) and 224.0.0.6 (DR/BDR); blocking 224.0.0.10 has no effect on OSPF.
• C - OSPF process IDs are locally significant only; two routers with different process IDs (e.g., router ospf 1 vs router ospf 99) will still form a neighbor relationship as long as everything else matches.

Memory tip: Think of OSPF adjacency like scheduling a meeting - if you and your neighbor disagree on when to meet (timer mismatch = D) or both show up claiming to be the same person (duplicate router ID = E), the meeting never happens.

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