Weird BGP Issue

Jason L. Weisberger jweis at
Wed Jul 22 02:53:33 UTC 1998

Lets say ISP X is peering with Tier One's A and B, as well as buying
transit from them. ISP Y also peers with A and B, in another geographic
region. While all routes for Y are best through A for X, it seems that 75%
of X's routes have a return path of A for Y to reach X and the other 25%,
although annouced in the EXACT SAME MANNER, use B for the return.

ISPs A and B both claim they are distributing all of the routes in the
exact same manner, there is no difference between a route for one of my
/17's and there is for one of my /19's - yet some of my traffic is taking
a different return path than the rest of it. 

Assymetry is cool and all, but this boggles the mind.


I announce and to A and B.
ISP A and B annouce them to both to Y, making no discrimination between
the two routes.
ISP Y takes A for and B for

Why doesn't take the route from A as best is is best from A
or why doesn't it choose B over A, as the route appears

I'd like to understand this c.p. and not just find an answer that is
"prepend your routes to A and B will always be preferred."

Any ideas?

Jason Weisberger
Chief Technology Officer
SoftAware, Inc. - 310/305-0275

...but the wicked shall do wickedly...
--Daniel 12:10

More information about the NANOG mailing list