Common IGP Routing Protocol Issues, Causes, and Solutions
Layer 1 or 2 down
  • One of the causes for routes not being installed in the routing table is that Layer 1 or Layer 2 is down. Layer 1 or 2 could be down for several reasons.

This example shows two routers running OSPF between each other.

Debugs and Verification
Figure shows the output of show ip ospf interface command for Ethernet 0, which shows that the line protocol is down.

Solution
To correct this problem, the Layer 2 problem must be fixed by checking previously mentioned conditions. The solution could be as simple as plugging in a cable, or it could be as complex as bad hardware, in which case the hardware must be replaced.

Figure shows the output of show ip ospf interface command for Ethernet 0 after the Layer 2 problem is corrected.