Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.6 |  | Advertised network interface is down |

IGP routing protocols will not advertise any interface network address that is physically down. As soon as the advertised network comes back up, the routing protocol will start advertising it again in its updates.

This example shows two routers running RIP between each other.

Debugs and Verification
Figure shows that interface Ethernet 0 is down.

Figure shows the output from the debug ip rip command. In this debug, router R1 is not sending or receiving any RIP updates because Layer 2 is down. There is no output from this debug command because of this problem.

Solution
RIP, like other routing protocols, runs above Layer 2. RIP cannot send or receive any routes if Layer 2 is down. To correct this problem, Layer 2 or Layer 1 must be corrected.

Figure shows the interface Ethernet 0 after fixing the Layer 2 problem.

Figure shows the routing table of router R2.

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