By default, RIP updates are forwarded out all RIP enabled interfaces. However, RIP updates really only need to be sent out interfaces connecting to other RIP enabled routers.

For instance, refer to the topology in Figure 1. RIP sends updates out of its G0/0 interface even though no RIP device exists on that LAN. R1 has no way of knowing this and, as a result, sends an update every 30 seconds. Sending out unneeded updates on a LAN impacts the network in three ways:

Use the passive-interface router configuration command to prevent the transmission of routing updates through a router interface, but still allow that network to be advertised to other routers. The command stops routing updates out the specified interface. However, the network that the specified interface belongs to is still advertised in routing updates that are sent out other interfaces.

There is no need for R1, R2, and R3 to forward RIP updates out of their LAN interfaces. The configuration in Figure 2 identifies the R1 G0/0 interface as passive. The show ip protocols command is then used to verify that the Gigabit Ethernet interface was passive. Notice that the G0/0 interface is no longer listed as sending or receiving version 2 updates, but instead is now listed under the Passive Interface(s) section. Also notice that the network is still listed under Routing for Networks, which means that this network is still included as a route entry in RIP updates that are sent to R2.

Note: All routing protocols support the passive-interface command.

Use the Syntax Checker in Figure 3 to configure the LAN interface as a passive interface on R2 and R3.

As an alternative, all interfaces can be made passive using the passive-interface default command. Interfaces that should not be passive can be re-enabled using the no passive-interface command.