Hello David
Unfortunately, I don’t have a clear-cut answer for you that will explain all of the packets we see in the Wireshark capture. However, there are some characteristics of EIGRP and the underlying transport layer protocol it uses that can explain some of what we see.
First of all, keep in mind that 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. In this case, the unicast Update packets 3, 4, and 11 are likely part of a specific conversation between R1 and R2. These updates do not contain any new routes 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. See this post for more info about RTP and how it appears in Wireshark.
Now RTP may require that some packets be resent to achieve reliability based on its operation and functionality. More info about RTP can be found at this Cisco Press article. In addition, detailed information about how RTP behaves in EIGRP exchanges can be found in the relevant RFC for EIGRP.
You might 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.
So these describe just some of the logic of the mechanisms used by EIGRP, and may help to explain why we see what we see in our packet captures…
I hope this has been helpful!
Laz