- 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.