Re: Weird BGP Issue

[Tatsuya Kawasaki <tatsuya () giganet net>]

Dear Jason,

we have similar situation but it is rather complicate.
To answer your question, it may be wrong or you obviously
omit something but according to your explaination, you did not
mention anyhing about ISP Y's routing policy.
Assume that A and B does not do anything on your BGP how about 
ISP Y?  Did you talk to ISP Y about it? don't ask me why they do but
it is mostly likely ? or am I missing all ?
I assume  there is NO carrier btwn X and Y other than A and B.

In theory, someone else other than you  can change BGP route update
infomation, but I don't think it is the case or perhaps?

regards,

Tatsuya

P.S. please let me know if you hear better explanation.

かわさき
TK3197

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On Tue, 21 Jul 1998, Jason L. Weisberger wrote:

> 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.
>
> Basically:
>
> I announce 207.155.0.0/17 and 209.151.224.0/19 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 207.155.0.0/17 and B for 209.151.224.0/19
>
> Why doesn't take the route from A as best is 207.155.0.0/17 is best from A
> or why doesn't it choose B over A, as the 209.151.224.0/19 route appears
> better.
>
> 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