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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