what does the blank update of the eigrp mean?

Answered Question
Apr 29th, 2008
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there are two routers directly connnected,each of them network the shared link, i captured some blank eigrp update without any TLV,what does that mean?

Correct Answer by ruwhite about 8 years 12 months ago

It depends.... :-)


In reality, hellos and updates look identical, except a couple of bits, and a lot of network analysis tools mix them up. We worked with a couple of makers to straighten this out, but I'm not certain if they are all correct, etc.


Now, given they were really empty updates, and not hello packets.... There are a couple of times EIGRP will send an empty update. The first is during neighbor bringup, to verify unicast and multicast reachability between the neighbors before beginning to exchange routes. Another is when a router wants to bring a neighbor or set of neighbors down. EIGRP brings down a neighbor (sometimes) by sending an empty update with the initialization bit set. It's also possible that the empty update is really an end of table indicator, which tells the receiving peer that the sender has finished sending the local topology table. Finally, it's possible that the empty update is actually an acknowledgment, which just has a couple of bits set, and one TLV containing the serial number of the ack'd packet.


So, there are a lots of options here, it just depends on when the "empty" packet was sent, or rather what the state of the neighbors was when the packet was sent, etc.


:-)


Russ

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Correct Answer
ruwhite Tue, 04/29/2008 - 17:23
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It depends.... :-)


In reality, hellos and updates look identical, except a couple of bits, and a lot of network analysis tools mix them up. We worked with a couple of makers to straighten this out, but I'm not certain if they are all correct, etc.


Now, given they were really empty updates, and not hello packets.... There are a couple of times EIGRP will send an empty update. The first is during neighbor bringup, to verify unicast and multicast reachability between the neighbors before beginning to exchange routes. Another is when a router wants to bring a neighbor or set of neighbors down. EIGRP brings down a neighbor (sometimes) by sending an empty update with the initialization bit set. It's also possible that the empty update is really an end of table indicator, which tells the receiving peer that the sender has finished sending the local topology table. Finally, it's possible that the empty update is actually an acknowledgment, which just has a couple of bits set, and one TLV containing the serial number of the ack'd packet.


So, there are a lots of options here, it just depends on when the "empty" packet was sent, or rather what the state of the neighbors was when the packet was sent, etc.


:-)


Russ

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