Understanding OSPF Neighbor States
OSPF (Open Shortest Path First) establishes neighbor relationships through several states:
1️⃣ Down: No Hello packets received.
2️⃣ Init: Router receives Hello packets but its own Router ID is missing in the received packets.
3️⃣ 2-Way: Routers see each other in Hello packets. (Stable state for non-DR/non-BDR routers).
4️⃣ ExStart: Routers start exchanging LSAs but may face MTU mismatches.
5️⃣ Exchange: Routers send Database Description (DBD) packets to synchronize link-state databases.
6️⃣ Loading: Routers exchange Link-State Requests (LSRs) & Updates (LSUs) to fully sync.
7️⃣ Full: Routers are fully synchronized.
Analysis of the Answer Choices
✅ A. For non-DR and non-BDR routers on an Ethernet link, the 2-Way state is acceptable.
✅ B. The Init state indicates that the router has received Hello packets from its neighbors, but the received Hello packets do not contain the OSPF RID of the receiving router.
Correct: If an OSPF router receives a Hello but does not see its own Router ID, it stays in the Init state.
✅ C. The ExStart state indicates that two neighbor routers have inconsistent MTUs or the same OSPF Router ID (RID).
Correct: If MTUs mismatch or RIDs conflict, the DBD exchange fails, causing routers to remain in ExStart state.
❌ D. The Attempt state indicates that the router does not send unicast Hello packets to configured neighbors.
Incorrect:
In Attempt state, routers send unicast Hello packets to manually configured neighbors (common in NBMA networks).
If no Hello responses are received, they remain in Attempt state, but they DO send Hellos.
Real-World Application
OSPF Troubleshooting: Identifying routers stuck in Init or ExStart states can point to MTU mismatches or neighbor misconfigurations.
NBMA Networks (Frame Relay, DMVPN): Manually configured neighbors may stay in Attempt state if Hello packets are lost.
✅ Reference: Huawei HCIE-Datacom Study Guide – OSPF Neighbor States & Troubleshooting
