Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.7 |  | Passive interface |

The passive-interface command works differently with the different IP routing protocols that support it.

RIP/IGRP – With RIP and IGRP, routing updates are received, but are not sent.

EIGRP – With EIGRP, the router stops sending hello packets on passive interfaces. When this happens, EIGRP cannot form neighbor adjacencies on the interface, and routing updates can neither be sent nor received.

OSPF – With OSPF, routing information is neither sent nor received on a passive interface.The network address of the passive interface appears as a stub network in the OSPF domain.

This example shows two routers running IGRP between each other.

When an interface is defined as passive under IGRP, IGRP will receive updates on that interface but will not send any updates. The passive-interface command is used to avoid sending unnecessary updates to a neighbor that does not need to receive any IGRP updates, such as a small router at the edge. A simple default route is enough information for that router to talk to the outside world. It is important to use the passive-interface command only where needed. If not, undesired results might occur.

Debugs and Verification
Figure shows the output of the show ip protocols command, which shows the outgoing interface is defined as passive.

Figure shows the configuration of router R1, which shows that the outgoing interface is defined as passive.

Solution
Figures and confirm that the interface Ethernet 0 is defined as passive, so router R1 is not sending any updates on Ethernet 0. Sometimes, it is desirable for some networks to be advertised and others to be filtered. In this situation, a distribute-list out would be a better solution.

In this example, the assumption is that the passive-interface was configured by mistake, and this command needs to be removed to solve this problem. Figure shows the new configuration to solve this problem.

Figure shows the routing table entry on router R2 after fixing the problem.

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

- show ip protocols
- debug ip routing
- debug ip rip
- debug ip igrp transactions
- debug ip eigrp
- debug isis adj packets
- debug isis update-packets