with a flap flap here and a flap flap there...
Sean M. Doran
smd at clock.org
Wed Feb 4 22:20:12 UTC 1998
Marc Slemko <marcs at znep.com> 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:
C
/|
A--B< |
\|
D
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.
Sean.
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