rfd

Tue Dec 18 21:54:19 UTC 2018

Dear Steve,

No worries, I have not forgotten the transitive properties of the
LOCAL_PREF BGP Path Attribute! :-) You are right that any LOCAL_PREF
modifications (and the attribute itself), are local to the Autonomous
System in which they were set, but the effects of such settings can
percolate further into the routing system.

A great example is the "BGP Graceful Shutdown" mechanism (science
partially documented in https://tools.ietf.org/html/rfc6198, actual
specification here https://tools.ietf.org/html/rfc8326). What is
interesting is that by considering a path (any path, could be flapping)
my network will propagate alternative paths to my neighboring networks,
or possibly even *withdraw* my announcement in favor of alternative
(stable?) paths via competitors.

By attaching a lower LOCAL_PREF value to a given path for a period of
time as a 'penalty' for flapping, I suspect the visiblity of that
flapping will be greatly reduced. This of course doesn't hold true when
the only origin of the path is flapping, but in many flapping cases I
triaged it was clear that only one out of many links was the root of the
flapping.

I'm not sure I share your concerns about scale, it appears that so far
we seem to be doing just fine without "route flap dampening, penalty
type: suppress". No customers ask for it, in fact many are relieved we
don't use it. None of our peering partners ask for it either. When we
see oscillating paths we reach out to the offending party and ask them
to fix it, or take unilateral action within a specific time frame.

Kind regards,

Job