Cisco IOS uses the router eigrp asn command (where asn is an autonomous system number [ASN]), plus one or more network net-id wildcard-mask subcommands, to enable EIGRP on the router and on router interfaces. The rules for these commands are as follows:
1. Neighboring routers’ router eigrp asn commands must be configured with the same ASN parameter to become neighbors.
2. Cisco IOS enables only EIGRP on interfaces matched by an EIGRP network command. When enabled, the router does the following:
a. Attempts to discover EIGRP neighbors on that interface by sending multicast EIGRP Hello messages
b. Advertises to other neighbors ...