Understanding OSPF Neighbor States
OSPF (Open Shortest Path First) establishes neighbor relationships through several states:
1## Down: No Hello packets received.2## Init: Router receives Hello packets but its own Router ID is missing in the received packets.3## 2-Way: Routers see each other in Hello packets. (Stable state for non- DR/non-BDR routers).4## ExStart: Routers start exchanging LSAs but may face MTU mismatches.5## Exchange: Routers send Database Description (DBD) packets to synchronize link-state databases.6## Loading: Routers exchange Link-State Requests (LSRs) & Updates (LSUs) to fully sync.7## 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.
* Correct: Non-DR/Non-BDR routers remain in 2-Way state since they don't need to form full adjacency.
# 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