bgp convergence problem

Song Li refresh.lsong at gmail.com
Tue May 6 11:42:08 UTC 2014


Hi Dennis,

I think there are two possible convergence results:

1/ AS3 accepted route 16.1/16(2 4 5) from AS1, then it will withdraw 
announce of 16.1/16(5) towards AS1. And AS1 will remain 16.1/16 (2 4 5).

2/ AS1 accepted route 16.1/16(3 5) from AS3, then it withdraw 16.1/16(2 
4 5), and AS3 will remain 16.1/16(5).

I simulated this case in GNS3, and only got the first kind of result, i 
don't know why?

Song

于 2014/5/6 18:13, ISP Services 写道:
> 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!
>>
>
>


-- 
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



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