bgp convergence problem

Song Li refresh.lsong at gmail.com
Tue May 6 03:58:58 UTC 2014


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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