bgp convergence problem

Wed May 7 17:28:46 UTC 2014

Operationally speaking, AS1 should not be leaking routes from one upstream to the other.  Bad route policy.  Also, AS3 should not accept routes from AS1 that don't belong to it.  Customer router filtering would prevent this.

> -----Original Message-----
> From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Song Li
> Sent: Monday, May 5, 2014 11:59 PM
> To: NANOG
> Subject: bgp convergence problem
> 
> Hi everyone,
> 
> 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.
> 
> Thanks!
> 
> --
> Song Li
> Room 4-204, FIT Building,
> Network Security,
> Department of Electronic Engineering,
> Tsinghua University, Beijing 100084, China Tel:( +86) 010-62446440
> E-mail: refresh.lsong at gmail.com