[c-nsp] Reasons for "random" ISIS flapping?

[Adam Vitkovsky]

As far as the 'Down, hold time expired' vs ' Down, neighbor forgot us' goes.

To underpin my theory I had to check my notes to confirm and yes indeed the
LAN IIHs contain IS neighbor TLV 6. 

So in case Router-A does not see the hellos for 800ms or 1s it will thorn
down the adjacency on his side and will try to reinitialize it by sending
the IIH but this time with an empty TLV 6. 
When e.g. Router-B will receive such an IIH it will realize that its
neighbor has forgot him and will thorn down the adjacency to participate in
the new adjacency formation procedure with Router-A. 

So the question seems to be why Router-A stops receiving IIHs or BFD hellos
for a split second from time to time while no one else notices any problems
until they learn that Router-A dropped the adjacency. 

Maybe some ingress buffers overruns or some CoPP. 

adam