bgp convergence problem

Randy Bush randy at psg.com
Tue May 6 15:27:03 UTC 2014


> 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


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