Subject: BFD shares fate with the Control Plane and unplanned restart

From: "Yin Lee" <[email-masked]>
Date: Sun, 14 Sep 2008 01:43:36 -0400

I have a question wrt the following sections in draft-ietf-bfd-generic-04
"4.3.2. BFD Shares Fate with the Control Plane

   If BFD shares fate with the control plane on either system (the "C"
   bit is clear in either direction), a BFD session failure cannot be
   disentangled from other events taking place in the control plane.  In
   many cases, the BFD session will fail as a side effect of the restart
   taking place.  As such, it would be best to avoid aborting any
Graceful Restart taking place, if possible (since otherwise BFD and
Graceful Restart cannot coexist.)  ...

4.3.2.2. Control Protocols Without Planned Restart Signaling
   Control protocols that cannot signal a planned restart depend on the
   recently restarted system to signal the Graceful Restart prior to the
   control protocol adjacency timeout.  In most cases, whether the
   restart is planned or unplanned, it is likely that the BFD session
   will time out prior to the onset of Graceful Restart, in which case a
   topology change SHOULD be signaled in the control protocol as
   specified in section 3.2."

Q: If the 'C' bit is cleared,  "it would be best to avoid aborting an
Graceful Restart taking place, if possible (since otherwise BFD and
   Graceful Restart cannot coexist.)" ,
why is it "whether the
   restart is planned or unplanned, it is likely that the BFD session
   will time out prior to the onset of Graceful Restart, in which case a
   topology change SHOULD be signaled in the control protocol
 specified in section 3.2"

Why signal topology change if the router knows a GR is taking place
(for a planned restart) and the BFD is fate sharing with the control
plane?

Thanks
Cheng-Yin

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From: "Vishwas Manral" <[email-masked]>
Date: Sun, 14 Sep 2008 17:38:16 +0530

Hi Cheng-Yin,

You  are nearly right. In case if we know beforehand we should not
have BFD signal topology change in case of planned restart.

However note that  Section 4.3.2.2 is for protocols which do not
support planned restart signalling the other end BFD will not know
there is a restart or not. So BFD will signal the topology change
before the restart signalling starts. Note the last section of 10.1.2,
that will clarify your doubt.

Thanks,
Vishwas