rted into the routing table, overwriting the previous entry. This means that each time Router B receives an update from Router A or Router C, it will change its routing table to show that network 10.0.0.0 is behind the router from which it received the update. Router B believes the last update it receives Router A Router C advertises advertises 10.0.0.0 10.0.0.0 192.168.1.0/24 192.168.2.0/24 10.10.10.0/24 10.20.20.0/24 Figure 10-9. RIP classful design problem You might be tempted to say that the networks behind Routers A and C in Figure 10-9 are different, but from RIP's point of view, you would be wrong. Technically, the networks behind Routers A and C are the same. They are both part of the 10.0.0.0/8 network. The routers are connected to different subnets within the 10.0.0.0/8 network, which is why RIP has a problem with the design. Specific Routing Protocols The only other type of network that RIP understands is a host network. RIP can advertise a route for a /32 or 255.255.255.255 network. Because RIP does not include subnet masks in its updates, a route is determined to be a host route when the address of the network is other than a normal network address. Routing protocols are configured in IOS using the router command. The protocol name is included in the command, which puts the router into router configuration mode: Router-A (config)# router rip Router-A (config-router)# On modern routers that support RIPv2, if you wish to use RIPv1, you must specify it explicitly in the router configuration because RIPv2 is used by default: router rip version 1 By default, no interfaces are included in the routing protocol. This means that no interfaces will have routing updates sent on them, and any routing updates received on the interfaces will be ignored. To enable interfaces in a routing protocol, you specify the networks that are config- ured on the interfaces you wish to include. This task is accomplished with the network command in router configuration mode: Router-A (config)# router rip Router-A (config-router)