Which of the following reasons may prevent OSPF neighbor relationships from being established? (Multiple choice)

Question

Options

• AThe two directly connected interfaces use the p2p network type, but the IP packet with the destination IP address 224.0.0.6 is refused to be transmitted.
• BThe dr-priority of the two directly connected interfaces is set to 0.
• CThe two routers use the same Router ID as each other.
• DThe sending intervals of Hello messages between the two routers are inconsistent.
• EThe two routers are configured with the same Router ID.
• FThe two directly connected interfaces use different network types.

• A
  4% (1)
• B
  7% (2)
• C
  4% (1)
• E
  86% (24)

Explanation

Options E and F are correct because OSPF has strict requirements for neighbor formation: each router must have a unique Router ID (E - duplicate IDs cause Hello packets to be rejected), and both interfaces must share the same network type (F - a mismatch such as broadcast vs. point-to-point results in incompatible DR/BDR election logic and timer expectations, blocking the neighbor state machine).

Why the distractors fail:

• A - On point-to-point networks, OSPF uses 224.0.0.5 (AllSPFRouters) for Hellos, not 224.0.0.6 (AllDRRouters); blocking 224.0.0.6 is irrelevant to p2p neighbor establishment.
• B - A DR priority of 0 only disqualifies a router from DR/BDR election; Hello exchange and 2-way neighbor state can still be reached normally.
• C - "Using the same Router ID as each other" implies the two routers have swapped IDs (A uses B's and B uses A's), so their IDs are actually distinct - no collision exists.
• D - Inconsistent Hello intervals affect the local sending rate but the Dead interval governs when a neighbor is declared down; in many implementations a Hello-interval mismatch alone does not block the neighbor relationship from forming (the Dead interval is the binding check).

Memory tip: Think "ID uniqueness + type unity" - OSPF neighbors need both a unique identity (no duplicate Router IDs) and a common interface personality (same network type); violate either and the handshake never completes.

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