bgp convergence problem

ISP Services nanog at isp-services.nl
Tue May 6 10:13:16 UTC 2014


Hi Song Li,

As far as I know there are 2 mechanisms that should prevent this 
situation you describe from happening:

- Not advertising routes that are not in the RIB
Once AS1's peering with AS3 comes back up, the route through AS3 is 
learned and preferred. Therefore the route via AS2 is purged from the 
RIB. Once it is no longer in the RIB, AS1 cannot announce that path anymore.

- AS Path loop prevention
If AS1 still leaks the prefix to AS3, it can only announce the active 
path which points to AS3 itself. Therefore AS3 will see a prefix with 
its own ASN in the path and (should) drop the prefix. Crisis avoided.

My textbook knowledge is a bit rusty though..

Dennis Hagens

Song Li schreef op 5/6/14 5:58 AM:
> 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!
>




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