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
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 = = = = = = Business Network Telecom (BNT) ビジネスネットワークテレコム株式会社 〒111-0053 東京都台東区浅草橋3-8-5 31山京ビル6階 TEL 03-5687-3945 FAX 03-5687-6009 http://www.giganet.net 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
omit something but according to your explaination, you did not mention anyhing about ISP Y's routing policy.
Y's routing policy should effect both blocks in the same way however, and not be an issue - so long as they haven't decided to specifically route the block strangely. I'm actually dumbing down this problem as we obviously have more than just 2 peers and certainly more than 1 AS-hop at play here, but if I can get an answer to why this is happening I can reason it out from there.
In theory, someone else other than you can change BGP route update infomation, but I don't think it is the case or perhaps?
Theoretically people are not tweaking the routes, atleast thats what I am told. Oh well, back to pounding my head against a wall and watching the History channel. -- Jason Weisberger Chief Technology Officer SoftAware, Inc. - 310/305-0275 ...but the wicked shall do wickedly... --Daniel 12:10
Dear Jason, On Wed, 22 Jul 1998, Jason L. Weisberger wrote:
omit something but according to your explaination, you did not mention anyhing about ISP Y's routing policy.
Y's routing policy should effect both blocks in the same way however, and not be an issue - so long as they haven't decided to specifically route the block strangely.
Not necessary but it should ...
I'm actually dumbing down this problem as we obviously have more than just 2 peers and certainly more than 1 AS-hop at play here, but if I can get an answer to why this is happening I can reason it out from there.
Now you are speaking. That is the problem though I don't know why. other politics, I meant policies. We have similar problems. some of our block is annouced one way and another block are annouced other way though we are not doing anything really special. What makes even worse that though for some BGP path is annouced but because of filtering, it is not really a path. :~< I can almost bet that someone on your AS_PATH knows what is going on your BGP annoucement. One of the way to solve the problem is ask everyone on AS_Path to see anyone is really doing something. That's I am intend to do for my case. Any better idea?
In theory, someone else other than you can change BGP route update infomation, but I don't think it is the case or perhaps?
Theoretically people are not tweaking the routes, atleast thats what I am told.
Oh well, back to pounding my head against a wall and watching the History channel.
-- Jason Weisberger Chief Technology Officer SoftAware, Inc. - 310/305-0275
...but the wicked shall do wickedly... --Daniel 12:10
Tatsuya
participants (2)
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Jason L. Weisberger -
Tatsuya Kawasaki