7.1 Controlling Routing Updates
7.1.1 Controlling routing updates
Consider RTA in the figure, which is running a simple distance-vector routing protocol, RIP.

The network 10.0.0.0 command does two things. First, it tells RIP where to send and receive advertisements (which interfaces to send and receive updates on). The network 10.0.0.0 command enables RIP updates on all interfaces that have an IP address belonging to the 10.0.0.0 network (Bri0, S1, S2, and E0). Second, this command tells the RIP process what to advertise. All directly connected subnets belonging to the major network 10.0.0.0 are included in RIP updates, in addition to any dynamically learned routes. That means that RTA advertises the following networks: 10.1.1.0, 10.2.2.0, 10.3.3.0, and 10.4.4.0.

Unfortunately, the default behavior of RIP, or any routing protocol, may not be the best thing for an internetwork. Look again at the figure. Is it useful for RTA to send updates on all four interfaces?

Updating out E0 is a waste of resources. No other routers on the 10.4.4.0 subnetwork can receive the updates, so they serve no purpose. Meanwhile, sending updates creates slight (and needless) overhead and a potential security risk. (A malicious user could use a packet sniffer to capture routing updates and thus glean key network information.)

For these reasons, you can configure passive interfaces or route filters to control routing updates. Both strategies are discussed in the following sections.