Hello Samir
Yes it is. When we talk about EIGRP messages or packets, we often talk about each individual message type as a single discreet message. We get the impression that each one is sent separately and individually. However, part of designing network protocols involves making them efficient. This means that if multiple types of messages can all be packed into a single communication, it becomes much more efficient.
That’s what’s happening with the ACK. Actually, the ACK field within the EIGRP header exists in all message types. When an EIGRP router prepares an ACK, it waits for the next message of any type to be sent and populates the ACK field before sending it out. If no other EIGRP message is being sent at the time, then a Hello packet with the appropriate value in the ACK field is sent. The Hello packet is the smallest and most efficient message type to be sent to contain an ACK, so that’s how it is sent out.
EIGRP uses both unicast and multicast for its communication. What it will actually use in each case depends on things like topology, whether it’s the initial communication or subsequent communication, and the type of message being sent. The goal is to be as efficient as possible. The following Cisco documentation in PDF format contains a table (page 3) with the five types of packets and explains clearly when unicast and multicast are used.
In your particular case, can you share your PCAP file so we can see the specific exchange as well? Let us know.
I hope this has been helpful!
Laz