🚀 go-pugleaf

>> Assume that the line from A to B is working but the reverse direction
>> has a problem.  B will continue to receive keepalives and, worse,
>> routing information from A.  But when it tries to use that line to B the
>> packet gets dropped on the floor.  Meanwhile A has a potentially good
>> path to B but won't use it for lack of keepalives and routing updates.

cisco's serial keepalives measure the full serial link.  Keepalives
contain a sending sequence number, and the last sequence number
received from the other end of the link.  (e.g. mine's 5, I last saw
23 from you.)  This lets a router see if his transmit wire is broken,
as well as testing for a broken receive wire.  If the number last sent
from a router, and the number echoed from the far end are ever out of
sync by more than two, then the serial line is declared down.

In your scenario, B will continue to hear keepalives from A with A's
sequence number changing.  These packets also contain the last
sequence number that A heard from B before the line died.  Router B,
upon seeing that his echoed sequence number is out of date, will
declare the serial line down.  A will also declare the line down for
lack of any keepalives at all.

David Hampton
cisco Release Group