[171573] in North American Network Operators' Group
Re: bgp convergence problem
daemon@ATHENA.MIT.EDU (Randy Bush)
Tue May 6 13:55:27 2014
X-Original-To: nanog@nanog.org
Date: Tue, 06 May 2014 17:27:03 +0200
From: Randy Bush <randy@psg.com>
To: Song Li <refresh.lsong@gmail.com>
In-Reply-To: <53685E02.7060309@gmail.com>
Cc: North American Network Operators' Group <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
> I have one bgp convergence problem which confused me. The problem is as
> follows:
>
> +--------+
> | AS5 |
> +------+16.1/16 |
> | +-----+--+
> | |
> +---+--+ |
> | AS4 | |
> | | |
> ++-----+ |
> | |
> | |
> | |
> +-----+--+ +-+-----+
> | AS2 | | AS3 | 16.1/16 (5)
> | ISP | | ISP |
> +---^----+ +---^---+
> | |
> | +--------+ |
> +-----+ AS1 +----+
> |customer|
> +--------+
> 16.1/16 (2 4 5)
>
> AS1 multihomed to AS2 and AS3, for some reasons AS1 disconnect from AS3,
> and as a resutl the route to 16.1/16 will be 16.1/16 (2 4 5).
>
> After a while, the BGP seesion between AS1 and AS3 reestablished but
> AS1 leaks the route 16.1/16 (2 4 5) to AS3. At this point,
>
> 1/ AS1 will have two bgp routes for prefix 16.1/16: 16.1/16(2 4 5)and
> 16.1/16(3 5), according to shorter AS_PATH it will select 16.1/16(3 5)
> as best route.
>
> 2/ AS3 also have two bgp routes: 16.1/16(2 4 5) and 16.1/16(5),
> according to local_pref it will select 16.1/16(2 4 5).
>
> in this case, AS1 and AS3 select each other as the best route to AS5, i
> wonder which route will be the final best route after bgp convergence in
> AS1 and AS3.
this is a bgp wedgie. is it real and caught in the wild? tim would be
cheered.
randy
