BGP-confederation best path selection

[m.taras_u]

Hi, dear community!

Can someone help me to resolve question about best path selection in BGP-confederation. I have:

BGP routing table entry for 4.0.0.0/24, version 42

Paths: (4 available, best #2, table Default-IP-Routing-Table)

  Advertised to peer-groups:

     1001

  Advertised to non peer-group peers:

  1.1.3.2

  (65504) 204, (received & used)

    1.1.4.1 (metric 20) from 1.1.4.1 (192.168.1.5)

      Origin incomplete, metric 50, localpref 100, valid, confed-external

      Community: 1001:1001

  (65501 65504) 204, (received & used)

    1.1.1.1 (metric 20) from 1.1.1.1 (192.168.1.1)

      Origin incomplete, metric 50, localpref 100, valid, confed-external, best

      Community: 1001:1001

  (65502 65501 65504) 204, (received & used)

    1.1.2.1 (metric 20) from 1.1.2.1 (192.168.1.3)

      Origin incomplete, metric 50, localpref 100, valid, confed-external

      Community: 1001:1001

  (65501 65504) 204, (received & used)

    56.1.2.90 (metric 20) from 1.1.3.2 (192.168.1.8)

      Origin incomplete, metric 50, localpref 100, valid, confed-internal

      Community: 1001:1001

And I can't figure out why best path is selectin not according to as-path, but to router-id...

Could someone help me?:)

Thanks!

[Giuseppe Larosa]

Hello Taras,

the confederation mini AS portion of the path is ignored. More specifically:

>>The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the             AS_PATH length.

see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#background

The reason is that the mini AS in confederation are kept in a separate field/attribute named

AS_CONFED_SEQUENCE (that describes the paths within the BGP confederation only),

  in order to be easily stripped toward a true eBGP neighbor that sees the public AS number as the peer-as.

Hope to help

Giuseppe