Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.3 |  | 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. If this is the case, it is not a routing protocol problem. Layer 1 or 2 could be down for several reasons. The following is a list of the most common things to check if an interface or line protocol is down:

- Unplugged cable
- Loose cable
- Bad cable
- Bad transceiver
- Bad port
- Bad interface card
- Layer 2 problem at the Telco in the case of a WAN link
- Missing clockrate statement in the case of back-to-back serial connection

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.

Figure shows the output of debug ip igrp transactions command. The output shows that the router is not sending or receiving any IGRP updates because Layer 2 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.

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

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