I'm trying to get a better feel for the dynamics of some "maybe-necessary" BGP routing traffic, and had a few questions: Under what circumstances will BGP send an update (of any sort) to a peer when there is an internal failure that does _not_ result in the complete isolation of a prefix? For example: __ AS1-rtr1 ---*-- AS2-rtr1 --\ / | -- AS3 10/8 -X- AS1-rtr2 ---*-- AS2-rtr2 --/ ^ |--link goes down at X In this case, AS1 is announcing 10/8 to its peer, AS2. An internal link within AS1 goes down. Are there any circumstances under which AS2 will announce some kind of change to AS3? My thoughts: * AS1 might announce a MED change to AS2, and AS2 might propagate that to AS3 in some unknown way (manually?) * AS2 would see the path to AS1 change, and would change its announced MED accordingly * For some reason, AS1 might announce a withdraw on 10/8, and then re-announce it. Is this possible, and under what circumstances would this happen? * Other things? I feel like the answers might depend on networks using ibgp with route reflectors vs. full mesh, or using a different interior routing protocol. In a related situation, let's say AS1 did a 'clear ip bgp' on AS1-rtr2, which was the preferred link for AS2 to get to 10/8. Would AS2 propagate any internal BGP announcements to AS3? (Same kinds of reasons as above, but this time AS1-rtr2 actually stopped announcing anything to AS2) I'm not certain of the right framework in which to think about the answers to these questions, alas. Clues from the operational side would be very welcome indeed. -Dave -- work: dga@lcs.mit.edu me: dga@pobox.com MIT Laboratory for Computer Science http://www.angio.net/
I know that at least one vendor had a bug whereby when bestpath changed and the med also changed it did not always propogate the med change. - jared s74182 On Tue, Apr 16, 2002 at 08:11:41PM -0400, David G. Andersen wrote:
I'm trying to get a better feel for the dynamics of some "maybe-necessary" BGP routing traffic, and had a few questions:
Under what circumstances will BGP send an update (of any sort) to a peer when there is an internal failure that does _not_ result in the complete isolation of a prefix?
For example:
__ AS1-rtr1 ---*-- AS2-rtr1 --\ / | -- AS3 10/8 -X- AS1-rtr2 ---*-- AS2-rtr2 --/ ^ |--link goes down at X
In this case, AS1 is announcing 10/8 to its peer, AS2. An internal link within AS1 goes down. Are there any circumstances under which AS2 will announce some kind of change to AS3? My thoughts:
* AS1 might announce a MED change to AS2, and AS2 might propagate that to AS3 in some unknown way (manually?) * AS2 would see the path to AS1 change, and would change its announced MED accordingly * For some reason, AS1 might announce a withdraw on 10/8, and then re-announce it. Is this possible, and under what circumstances would this happen? * Other things? I feel like the answers might depend on networks using ibgp with route reflectors vs. full mesh, or using a different interior routing protocol.
In a related situation, let's say AS1 did a 'clear ip bgp' on AS1-rtr2, which was the preferred link for AS2 to get to 10/8. Would AS2 propagate any internal BGP announcements to AS3? (Same kinds of reasons as above, but this time AS1-rtr2 actually stopped announcing anything to AS2)
I'm not certain of the right framework in which to think about the answers to these questions, alas. Clues from the operational side would be very welcome indeed.
-Dave
-- work: dga@lcs.mit.edu me: dga@pobox.com MIT Laboratory for Computer Science http://www.angio.net/
-- Jared Mauch | pgp key available via finger from jared@puck.nether.net clue++; | http://puck.nether.net/~jared/ My statements are only mine.
participants (2)
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David G. Andersen -
Jared Mauch