7.1 Controlling Routing Updates
7.1.2 Passive interfaces
You can 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 (that is, RIP, IGRP, EIGRP, OSPF, and IS-IS). The syntax of this command is as follows:

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

You can configure RTA's E0 as a passive interface, as shown in Figure .

You can also configure WAN interfaces as passive interfaces, to prevent the sending of updates to link partners. You can 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 and will result in an unnecessarily large bill from the provider. Instead, a static route can be configured on both routers with RTA's Bri0 configured as a passive interface.

Note that for RTA to update the other routers (RTY and RTZ) about the route to 172.16.1.0, RTA 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 chapter.

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 via a passive interface. In EIGRP and OSPF, the router stops sending hello packets on passive interfaces. When this happens, the router can not form neighbor adjacencies, and thus can not send and receive routing updates on the interface. You will see later in this chapter that the passive effect can be achieved for an EIGRP interface (without preventing adjacency relationships) by using the distribute-list command.