Re: [Idr] WG LC for draft-ietf-bgp-route-oscillation-stop (4/6 to 4/20)

Daniel Walton <dwalton@cumulusnetworks.com> Thu, 16 April 2015 13:44 UTC

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Date: Thu, 16 Apr 2015 09:44:47 -0400
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From: Daniel Walton <dwalton@cumulusnetworks.com>
To: "Dongjie (Jimmy)" <jie.dong@huawei.com>
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Cc: idr wg <idr@ietf.org>, "dbrungard@att.com" <dbrungard@att.com>, Susan Hares <shares@ndzh.com>
Subject: Re: [Idr] WG LC for draft-ietf-bgp-route-oscillation-stop (4/6 to 4/20)
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>
> “For the sake of scalability the advertisement of multiple paths should be
> limited to those prefixes which are affected by MED-induced route
> oscillation in a network carrying a large number of alternate paths.”
>
>
>
> How do the RRs or confederation ASBRs identify those prefixes affected by
> MED-induced oscillation?  Is there some rules or it is operator
> configuration?
>

The easiest way is to monitor how often the bestpath changes for a prefix.
If the bestpath for a prefix changes at some consistent interval that is a
strong indicator of MED churn.  The details for how an implemenation would
automaticaly do this are beyond the scope of the draft.  If you want to do
it manually the easiest way is to do "show ip route | grep " 00:00" which
will show you any route that is less than a minute old.  Do that a few
times and see which prefixes keep popping up.  This of course assumes that
the box you are on displays how long the route has been in the RIB :)

As for what that churn interval typically is, it varies depending on the
network.  When we first found MED churn we often saw an interval of 60s
becuase IOS would recalculate all bestpaths as part of the BGP Scanner
process which kicked off every minute by default.  Eventually the bestpath
recalc was removed from scanner and made event-driven so then the churn
would be constant...or as fast as the MRAI timers in the network would
allow.

Daniel