EIGRP - Reliable Multicast

EIGRP uses the Cisco proprietary Reliable Transport Protocol or RTP for communication between EIGRP routers. This protocol has as part of its operation, a mechanism known as reliable multicast.

EIGRP will send its update message initially using multicast. If the EIGRP router doesn't get a reply from a neighbor via the multicast, it then uses unicasts to resend the same data. If it does not get a reply from the neighbor after 16 unicast attempts, it declares the router dead. This process is known as reliable multicast.