ExtremeSwitching (EXOS)

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BGP AS-PATH preferred route

  • 1.  BGP AS-PATH preferred route

    Posted 12-15-2017 13:39
    Hello, all!

    Does anybody faced with issue, BGP routes with the same Weight and LPref and Summit is choose preferred route with larger AS-PATH?
    EXOS 15.6.2

    For example:
    X670G2 # sh bgp
    routes detail network
    123.123.123.0/24

    Routes:
    Route: 123.123.123.0/24, Peer 1.2.3.4
    Origin IGP, Next-Hop 1.2.3.4, LPref 500, MED 0
    Weight 3000,
    As-PATH: ( 45002 )
    Community: 32808:1 32808:2 32808:3


    Route: 123.123.123.0/24, Peer 5.6.7.8, BEST, Active
    Origin IGP, Next-Hop 5.6.7.8, LPref 500, MED 0
    Weight 3000,
    As-PATH: ( 45001 45002 )

    Community: 32808:1 32808:2 32808:3

    Thank you!


  • 2.  RE: BGP AS-PATH preferred route

    Posted 12-16-2017 06:58
    Hello Alexandr,

    Inside the BGP onfederation, the best path selection cannot be calculated with AS PATH length attribute. This can be taken care only by adding addition configuration like Local preference or Weight.
    In your output shortest patch is selected based on Lowest Next hop IP address.

    Best Regards,
    Nikolay


  • 3.  RE: BGP AS-PATH preferred route

    Posted 12-16-2017 22:18
    To be more precise:

    https://tools.ietf.org/html/rfc5065#section-5.3

    "When comparing routes using AS_PATH length, CONFED_SEQUENCE and CONFED_SETs SHOULD NOT be counted."

    If you don't remember what is CONFED_SEQ and CONFED_SET: https://tools.ietf.org/html/rfc5065#section-3

    So, AS-Path is still used, but not with Member-AS. And you were looking only at member-AS, as the "()" stresses it in your output.


  • 4.  RE: BGP AS-PATH preferred route

    Posted 12-18-2017 00:56
    Hi Alexandr,

    Just wondering if the next hop (1.2.3.4) is reachable (active) or not.


  • 5.  RE: BGP AS-PATH preferred route

    Posted 12-18-2017 06:07
    Thank you all for help!!!


  • 6.  RE: BGP AS-PATH preferred route

    Posted 12-18-2017 00:56
    Hello, David!

    Both peers - 1.2.3.4. and 5.6.7.8 is reachable.
    If one of them was unavailable, there would be no questions 🙂

    Thank you!