Subject: Graceful Restart and BFD

From: Les Ginsberg <[email-masked]>
Date: Tue, 26 Oct 2004 23:51:16 -0700

Dave(s) -

Section 7.4 of draft-ietf-bfd-v4v6-1hop-00.txt discusses "Interactions with 
OSPF and IS-IS with Graceful Restart". The last paragraph of this section says:

    "If BFD shares its fate with the control plane on either system
    (either the local system shares fate with the control plane, or the
    remote system is transmitting BFD packets with the C bit set to
    zero), it is not useful during graceful restart, as the BFD session
    is likely to fail regardless of the state of the forwarding plane.
    In this situation, if a BFD session fails while graceful restart is
    taking place (or if the BFD session failure triggers a graceful
    restart event), the graceful restart SHOULD be allowed to complete
    and the topology change should not be made visible to the network as
    outlined in section 7.3."

This text recommends a behavior which is unsupportable in many cases. In 
order for this recommendation to be supported, the router which is about to 
restart MUST provide an indication to its neighbors that a planned graceful 
restart(GR) is about to happen ("I'll be back"). As "I'll be back" is NOT 
part of the IS-IS GR extensions, this CANNOT be supported by IS-IS. In 
addition, it cannot be supported by either protocol in cases of unplanned 
restart.

There are strategies an implementation could employ to deal with the 
control plane dependency issue, none of which conform to your 
recommendation ("if a BFD session fails while graceful restart is taking 
place..."). For example:

1)BFD could be administratively disabled prior to the start of GR.This 
works only for planned restarts, not unplanned restarts.
2)BFD failures could be ignored at all times when the remote end indicates 
control plane dependency ( C bit = 0). This effectively disables BFD in 
that direction.

Exactly what you want to say about this is up to you - but the current text 
should definitely be revised.

    Les