EIGRP - Update message exchange process
When implementing the EIGRP routing protocol, EIGRP enabled routers will create neighbor adjacencies by exchanging EIGRP messages. Initially, they will exchange hello messages and then will start sending updates containing network information.
The way these initial updates are exchanged depends on several factors.
EIGRP operates both using multicast and unicast. Initially, a prefix will be advertised using multicast destination addresses so that it can be advertised to all neighbors in a single go. Unicast updates are sent in response to queries or to provide specific updates to a particular neighbor.
There may be cases where updates are resent but they do not contain any new routes. This may occur because they might be part of a convergence process where no new routes are available yet, or they could be part of the EIGRP Reliable Transport Protocol (RTP) acknowledging receipt of an update.
RTP (not to be confused with Real-time Transport Protocol) is a Cisco proprietary transport layer protocol that is used exclusively for the exchange of EIGRP packets.
In addition, we may see multiple Update packets with the same routes due to the Reliable Multicast feature of EIGRP. If the EIGRP router doesn’t get a reply from a neighbor via the multicast, it will use 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 and may be behind some of the repeated updates.
Links:
https://forum.networklessons.com/t/detailed-look-at-eigrp-neighbor-adjacency/911/52?u=lagapides
https://forum.networklessons.com/t/eigrp-packet-how-is-it-packaged-for-network-distribution/14162/2