Hmmm.... The second packet is a multicast, and EIGRP doesn't send multicast packets with the ack piggybacked on to them. EIGRP should always send an ack unicast. Technically, this is because the actual number sent to identify the packet is a sequence number, and the sequence numbers each peer is using can overlap (does that make sense?), so there's no way to know which neighbor's sequence number is being acked if the ack is carried in a multicast....
Was there a hello in the middle that you could have missed? An ack looks just like a hello, it just includes a seperate tlv that most protocol analyzers don't do a good job at processing (unfortunately).
It _shouldn't_ matter what code you are running--EIGRP's reliable transport has worked pretty much the same way since about 10.3.... Anyway, looking at it in the lab, I see this on the router receiving the update about the new route:
00:16:31: EIGRP: Received UPDATE on Serial0/0 nbr 126.96.36.199
From the bottom to the top.... Since the event log is upside down:
-- The packet comes in, and is classed as an update
-- The update is dissected, and DUAL runs to determine what we need to do about it.
-- DUAL determines this is a new route, so we just install it, and set up a poison reverse.
-- The transport code pulls out all the neighbors on the interface we received the update on, and decides that clears the entire list of neighbors this poison needs to go to, so the poison is "squashed."
-- The ack is queued, and then sent.
So, you do seem to be running something older on the other routers, since you are seeing the poison on the wire. But, I don't know of any time we've ever accepted a multicast poison update as an ack for a packet we just sent. We kindof do this at startup, but it's still a unicast, and the ack is piggybacked onto the poison, which is why I ask about the startup case.
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