Troubleshooting OSPF Neighbor Relationships
This section discusses the problems related to establishing OSPF neighbor relationships. OSPF neighbor relationship problems can be of any type. Sometimes, the neighbor list is empty (that is, an OSPF neighbor might not even see the Hellos from each other). Sometimes, the problem is that the neighbor is stuck in a specific state. Recall from Chapter 8, "Understanding Open Shortest Path First (OSPF)," that the normal state of an OSPF neighbor is FULL. If the state is something other than FULL for a long period of time, this indicates a problem.
This section comes first because this is the most important step in using the OSPF protocol. If no neighbor relationships are established or the neighbors are stuck in a state other than FULL, OSPF will not install any routes in the routing table. Therefore, it is very important in OSPF to make sure that the neighbors are up.
OSPF neighbor relationship problems can be of any of these types:
• The OSPF neighbor list is empty.
• An OSPF neighbor is stuck in ATTEMPT.
• An OSPF neighbor is stuck in INIT.
• An OSPF neighbor is stuck in 2-WAY.
• An OSPF neighbor is stuck in EXSTART/EXCHANGE.
• An OSPF neighbor is stuck in LOADING.
None of the states mentioned in this list is an indication of a problem, but if a neighbor is stuck in one of these states for a long time, this is a problem and must be corrected; otherwise, OSPF will not function properly.
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This is the most common problem in OSPF neighbor relationships. The most common causes are related to either misconfiguration or lack of configuration. If the neighbor list is empty, it will not even proceed to form OSPF neighbor relationships.
The most common possible causes of this problem are as follows:
• OSPF is not enabled on the interface.
• The interface is defined as passive under OSPF.
• An access list is blocking OSPF Hellos on both sides.
• A subnet number/mask has been mismatched over a broadcast link.
• The Hello/dead interval has been mismatched.
• The authentication type (plain text versus MD5) has been mismatched.
• An authentication key has been mismatched.
• An area ID has been mismatched.
• Stub/transit/NSSA area options have been mismatched.
• An OSPF adjacency exists with secondary IP addressing.
• An OSPF adjacency exists over an asynchronous interface.
• No network type or neighbor is defined over NBMA (Frame Relay, X.25, SMDS, and so on).
• The frame-relay map/dialer map statement is missing the broadcast keyword on both sides.
Figure 9-1 shows two routers running OSPF between each other. The output of show ip ospf neighbor shows an empty list. In a normal scenario, the output displays the OSPF neighbor status. This figure is used for most of the causes described in this section.
Continue reading here: OSPF Neighbor List Is Empty Cause OSPF Not Enabled on the Interface
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