OSPF neighbor relationship problems 1) The OSPF neighbor list is empty.2) An OSPF neighbor is stuck in ATTEMPT.3) An OSPF neighbor is stuck in INIT.4) An OSPF neighbor is stuck in 2-WAY.5) An OSPF neighbor is stuck in EXSTART/EXCHANGE.6) An OSPF neighbor is stuck in LOADING.

Neighbour list is empty1) OSPF is not enabled on the interface.2) Layer 1/2 is down.3) The interface is defined as passive under OSPF.4) An access list is blocking OSPF Hellos on both sides.5) A subnet number/mask has been mismatched over a broadcast link.6) The Hello/dead interval has been mismatched.7) The authentication type (plain text versus MD5) has been mismatched.8) An authentication key has been mismatched.9) An area ID has been mismatched.10) Stub/transit/NSSA area options have been mismatched.11) An OSPF adjacency exists with secondary IP addressing.12) An OSPF adjacency exists over an asynchronous interface.13) No network type or neighbor is defined over NBMA (Frame Relay, X.25, SMDS, and so on).14) The frame-relay map/dialer map statement is missing the broadcast keyword on both sides.An OSPF neighbor is stuck in ATTEMPTWe can see this state in NBM mode.Common possible causes are1) Misconfiguration neighbour statement2) Unicast broken on NBMAAn OSPF neighbor is stuck in INIT1) An access list on one side is blocking OSPF Hellos.2) Multicast capabilities are broken on one side (6500 switch problem).3) Authentication is enabled on only one side (virtual link example).4) The frame-relay map/dialer map statement on one side is missing the broadcast keyword.5) Hellos are getting lost on one side at Layer 2.An OSPF neighbor is stuck in 2-WAYWhile electing DR and BDR, priority is modified to zero.An OSPF neighbor is stuck in EXSTART/EXCHANGEThe most common possible causes of this problem are as follows:1. Mismatched interface MTU2. Duplicate router IDs on neighbors3. Inability to ping across with more than certain MTU size4. Broken unicast connectivity because of the following: Wrong VC/DLCI mapping in Frame Relay/ATM switch Access list blocking the unicast NAT translating the unicast5. Network type of point-to-point between PRI and BRI/dialerOSPF Neighbor Stuck in LOADINGThe most common possible causes of this problem are as follows:1. Mismatched MTU2. Corrupted link-state request packetTroubleshooting OSPF Route AdvertisementThe most common reasons for OSPF to not share the database information about a specific link are as follows:1. The OSPF neighbor is not advertising routes.2. The OSPF neighbor (ABR) is not advertising the summary route.3. The OSPF neighbor is not advertising external routes.4. The OSPF neighbor is not advertising the default route.OSPF Neighbor Is Not Advertising RoutesThe most common possible causes of this problem are as follows:1. OSPF is not enabled on the interface that is supposed to be advertised.2. The advertising interface is down.3. The secondary interface is in a different area than the primary interface.OSPF Neighbor (ABR) Not Advertising the Summary RouteThe most common possible causes of this problem are as follows:1. An area is configured as a totally stubby area.2. An ABR is not connected to area 0.3. A discontiguous area 0 exists.OSPF Neighbor Is Not Advertising External RoutesThe most common possible causes of this problem are as follows:1. The area is configured as a stub or NSSA.2. The NSSA ABR is not translating Type 7 into Type 5 LSA.OSPF Neighbor Not Advertising Default Routes1. The default-information originate command is missing.2. The default route is missing from the neighbor's routing table.3. A neighbor is trying to originate a default into a stub area.4. The NSSA ABR/ASBR is not originating the Type 7 default.Troubleshooting OSPF Route Installation1. OSPF is not installing any routes in the routing table.2. OSPF is not installing external routes in the routing table.OSPF Not Installing Any Routes in the Routing Table1. The network type is mismatched.2. IP addresses are flipped in dual serial-connected routers or a subnet/mask mismatch has occurred.3. One side is a numbered and the other side is an unnumbered point-to-point link.4. A distribute list is blocking the routes' installation.5. There is a broken PVC in a fully meshed Frame Relay network with the broadcast network type.OSPF Not Installing External Routes in the Routing Table1. The forwarding address is not known through the intra-area or inter area route.2. The ABR is not generating Type 4 summary LSAs.Troubleshooting Redistribution Problems in OSPF1. ASBR is not advertising redistributed routes.2. OSPF is not installing external routes in the routing table. (already discussed before)ASBR is not advertising redistributed routes.1. The subnets keyword is missing from the ASBR configuration.2. distribute-list out is blocking the routes.Troubleshooting Route Summarization in OSPFOSPF can use two types of summarization:1. Inter area summarization that can be done on the ABR2. External summarization that can be done on the ASBRTwo common problems related to summarization in OSPF are as follows:1. A router is not summarizing inter area routes.a. Area range command not configured on ABR2. A router is not summarizing external routes.a. Summary-address command is not configured on ASBR