bgp convergence problem

ISP Services nanog at isp-services.nl
Tue May 6 12:54:57 UTC 2014


I suggest you work your way down :-)

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Dennis Hagens

Song Li schreef op 5/6/14 1:42 PM:
> 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!
>>>
>>
>>
>
>




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