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| 8.1 |  | Controlling Routing Update Traffic |
| 8.1.2 |  | Passive interfaces |

Configure LAN interfaces as passive interfaces when enabling a routing protocol. A passive interface receives updates, but does not send them. The passive-interface command can be used with all IP interior gateway protocols. It can be used with RIP, IGRP, EIGRP, OSPF, and IS-IS. The syntax of the command is as follows:

> Router(config-router)#passive-interface type number

Configure the E0 interface on RTA as a passive interface.

Use the passive-interface command on WAN interfaces to prevent routers from sending updates to link partners.

There may be several reasons to prevent updates on the WAN. If RTA and RTX are connected by a dial-on-demand ISDN link, regular RIP updates will keep the link up constantly. This will result in an unnecessarily large bill from the provider. Instead, a static route can be configured on both routers with the Bri0 on RTA configured as a passive interface.

Notice that for RTA to update RTY and RTZ about the route to 172.16.1.0, it must be configured to redistribute static routes into RIP. The redistribute static command tells RIP to import the static routes into RIP and advertise them as part of a RIP update. Route redistribution is covered in more detail later in this module.

The passive-interface command works differently with the different IP routing protocols that support it. In OSPF, the network address of the passive interface appears as a stub network. OSPF routing information is neither sent nor received by way of a passive interface. In EIGRP and OSPF, the router stops sending Hello packets on passive interfaces. When this happens, the router cannot form neighbor adjacencies. Therefore, the router cannot send and receive routing updates on the interface. It will be seen later in this module that the passive effect can be achieved for an EIGRP interface by using the distribute-list command. This can be done without preventing adjacency relationships.

Figures through are animations that are designed to help the student gain a better understanding of a passive interface in routing.

| Lab Activity e-Lab Activity: Passive Interfaces This lab is to use a passive interface on a router.