er A EIGRP 200 10.20.20.0/24 Figure 10-11. Multiple EIGRP instances As with all IGPs, you list the interfaces you wish to include using the network com- mand. EIGRP, like RIP, will automatically convert a classless network into the classful equivalent. The difference is that with EIGRP, you can add an inverse subnet mask to make the entry more specific. The following commands add all interfaces with addresses in the 10.0.0.0 network to the EIGRP 100 process: Router-A(config)# router eigrp 100 Router-A(config-router)# network 10.0.0.0 But in the example in Figure 10-11, we'd like to add only the interface with the net- work 10.10.10.0/24. The subnet mask for a /24 network is 255.255.255.0, and the inverse subnet mask is 0.0.0.255 (inverse subnet masks are also called wildcard masks, and are discussed in Chapter 23). So, to add only this interface, we'd use the following network command: Router-A(config-router)# network 10.10.10.0 0.0.0.255 After executing this command, the running configuration will still contain the less- specific 10.0.0.0 network statement: router eigrp 100 network 10.10.10.0 0.0.0.255 network 10.0.0.0 Chapter 10: Routing Protocols Both commands will take effect. Be careful of this, as it can cause no end of frustra- tion. In this example, it will cause the interface E1/1 to be included in EIGRP 100, which is not what we want. We need to remove the less-specific network command by negating it: router eigrp 100 no network 10.0.0.0 A very good practice to follow is to enable only the specific interface you wish to add in any routing process that supports it. This can be done by specifying the IP address on the interface with an all-zeros mask. In our example, the command would be network 10.10.10.1 0.0.0.0. This prevents surprises should network masks change, or interfaces be renumbered. Thus, my preferred configuration for EIGRP on the router shown in Figure 10-11 would be: router eigrp 100 network 10.10.10.1 0.0.0.0 network 192.168.1.1 0.0.0.0 router eigrp 200 network 10.20.20.1 0.0.0.0 network 