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

  • From: Sean M. Doran
  • Date: Wed Feb 04 17:41:20 1998

Marc Slemko <[email protected]> writes:

> But the loop avoidance from having paths constrains you to the width of
> the Internet in terms of ASes.  It isn't completely a distance vector
> protocol.

Yakov Rekhter also made a similar observation.
I was caught up in rhetoric and was imprecise.
(I could also be wrong; it happens.)

BGP records paths and the loop avoidance scheme prevents
a count-to-infinity problem in a way slightly better than
RIP with split horizons does.

In a network like this:

        A--B< |

if A-B goes down in a split-horizons RIP network, there
can still be a count-to-infinity problem between C and D
announcing reachability to A.

BGP does not have this problem.

However, a withdrawal of a network from D can
cause A to see a transition from ABD to ABCD
to unreachable.   This effect is what was complained
about in the message I initally followed-up to.

BGP is also not formally a distance vector protocol
because the AS_PATH attribute is a trail of breadcrumbs
rather than a distance.   However, I argue that in common
practice, it is a distance, and offer up AS-path
prepending to affect path selection remotely as evidence.

> I guess I'm just suprised at how wide the Internet has grown and at the
> lack of noticeable public acknowledgment of the resulting problems.

I'm not so surprised by the first bit, and I think
I have become jaded about the second.