North American Network Operators Group

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Re: with a flap flap here and a flap flap there...

  • From: Sean M. Doran
  • Date: Wed Feb 04 13:34:58 1998

Marc Slemko <[email protected]> writes:

>   3333 6905 5623 1136 3300 7018 6478 1 701 814 7189 1691 852 6171 (history entry)
> Am I the only one that has a problem with this?  You withdraw a 
> route once, and it has flapped enough to be dampened in numerous 
> places.  I could have been half convinced that we had gone back to 
> distance vector routing seeing this going on...

BGP *is* a distance-vector routing protocol.

What you were witnessing was a counting-to-infinity
problem that is bounded by timers and toplogical constraints.

Welcome to the wonderful world of BGP == RIP.  

This is precisely why a mechanism which constrains
the amount of prefix transitions is so important:
magnification of flapping is predictable.

However, welcome to the wonderful world of lack of
filtering, too.  There is no apparent reason why a number
of those ASes should have reannounced your routes to each
other.  Each BGP peering (customer and peer alike) should
either explicitly allow announcements only from a list of
appropriate ASes, or explicitly disallow announcements
which have been through peers and (at least large) other
BGP-talking customers.  This is an Incredibly Smart Thing To Do.
On a Cisco, the _ operator is your best regexp friend...