Cisco EIGRP protocol allows different methods to form neighborship relations. Default method is form neighborship automatically via multicast (multicast address of 224.0.0.10 is used)

After an adjacency is formed ‘hellos’ are  sent using IP protocol 88 (eigrp). This means that;

1. an interface need to be IP enabled
2. both interfaces to a link need to have an ip address within the same subnet.

Alternative method of creating an neighborship relation is setting the neighborship ‘hard-coded’. This is done withing the eigrp routing context. Setting a neighborship ‘over’ an interface implicitly disables the ability to form adjacencies using multicast. Inbound EIGRP multicast packets are discarded and outbound are not send out that particular interface.

This alternative method can be used when you want to prevent a rogue router in your network inject all sorts of garbage subnets.

Off course a better way to set a md5 key on eigrp update exchanges.