BGP route update propagation questions

Wed Apr 17 00:34:00 UTC 2002

	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 at lcs.mit.edu                          me:  dga at pobox.com
>       MIT Laboratory for Computer Science           http://www.angio.net/

-- 
Jared Mauch  | pgp key available via finger from jared at puck.nether.net
clue++;      | http://puck.nether.net/~jared/  My statements are only mine.