Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.2 |  | Missing or incorrect network or neighbor statement |

When configuring or modifying a network it may become apparent that a route is missing from the routing table. There can be many reasons for this. One of the obvious things to check is to see whether the network statement under router configuration has been properly configured.

For IGP routing protocols, the network statement does two things:

1. Enables the routing protocol on interfaces with IP addresses that match the IP address in the network statement, giving the interface the capability to send and receive updates.
2. Advertises that network in its own updates to other routers.

This example shows two routers running OSPF between each other.

Debugs and Verification
The output of the show ip ospf neighbor command shows an empty list. In a normal working scenario, the output would display the OSPF adjacent neighbors.

With some protocols like OSPF and EIGRP, the routing protocols can be enabled on a per-interface basis using wildcard masks. Careful configuration of the wildcard mask is important so interfaces are not incorrectly included or incorrectly left out.

An obvious place to begin is by looking at the running-configurations on both routers by using the show running-config command. Figure shows the configuration of Router R2. The configuration shows that the network statement exists, but a closer look reveals that the wrong wildcard mask is used. The network statement is determined in OSPF in exactly the same way that an access list would be defined. The main idea here is to include the range of addresses in the area. The network statement of 131.108.0.0 with the wildcard mask of 0.0.0.255 will not cover 131.108.1.2. It covers only the range from 131.108.0.0 to 131.108.0.255, as indicated by the wildcard mask.

Figure shows the output of the show ip ospf interface command and that OSPF is not enabled on the Ethernet 0 interface of Router R2.

Depending upon the routing protocol, there are several other commands that can help troubleshoot this issue. The show ip protocols command will display which networks are originating from this router. The debug command can be used to verify whether the routing update is being sent or received, or if there are any mismatched timers, subnet masks, and so on:

- debug ip rip
- debug ip igrp events
- debug ip igrp transactions
- debug ip eigrp
- debug ip ospf events

Solution
The obvious solution is to correctly configure the network statements to enable the routing protocol on the appropriate interfaces. In regards to OSPF, there is a rare case where the configuration shows the correct mask and the OSPF neighbor list still appears empty. During network configuration under OSPF, a cut-and-paste of the OSPF configuration might create this problem. Therefore, it is always best to look at the output of show ip ospf interface for that specific interface and confirm whether OSPF is enabled on that interface. This specific problem can be corrected by reentering the network statement.

Figure shows the new configuration that fixes the OSPF network problem. In this example, the wildcard mask is 0.0.255.255, which means that it covers the range from 131.108.0.0 to 131.108.255.255

Figure shows the output of show ip ospf neighbor after applying the correct network mask. Beginning with Cisco IOS 12.0, the output of show ip ospf interface does not display anything if OSPF is not enabled on the interface.

Commands used in this example and some of the commands that can be used for other routing protocols include:

- show ip protocols
- show ip interface
- show ip interface brief
- show ip eigrp interfaces
- show ip eigrp neighbors
- show ip ospf interface
- debug ip routing
- debug ip rip
- debug ip igrp events
- debug ip igrp transactions
- debug ip eigrp
- debug ip ospf events
- debug isis adj packets
- debug isis update-packets