5.3 - BGP Peering

Hello everyone!

 

I just wondered what thoughts folks had on the use of the "no-prepend" switch at the end of the command, and whether it was necessary in this case.  I've personally never really had the occasion to use the local-as option, as when changing an AS number on a device, we've typically just coordinated a maintenance window agreeable to both parties and have done both sides at the same time.

 

Obviously the use somewhat depends on the topology, but in this case, where AS 100 is connected to AS 54 elsewhere, not using it shouldn't break connectivity, correct?  I believe it'd just make it so that R6 would not have an alternate path to the AS 54 prefixes should its frame link go down...

 

Thoughts?

 

Thanks!

 

--JB

Comments

  • I had the same question in my head while reviewing this solution .... if we dont use "no-prepend" at the end, routes will have AS 100 installed in the AS-PATH .. now assume this route will be advertised to the actuall AS 100 on R6 .. then R6 will NOT accept this routes if it sees it's own AS in AS-PATH (unless neighbor is configured to ignore local-as in AS-PATH).

     

    Dmitriy

  • Fair enough, I hadn't thought about this going over to R6, just was thinking locally.  :)

     

    Thanks for the reply.

  • the no-prepend is  not about breakin' the connectivityas what have been said

    it's  preventing SW1 from sending the local-as info int AS pathto other neighbors like if you check it on R1

    Rack1R1(config-if)#do sh ip bgp 28.119.16.0
    output omitted
         2        
      400 100 54 (without the 'no-prepend')<-------------------------------------------------
        132.1.17.7 from 132.1.17.7 (150.1.7.7)
          Origin IGP, localpref 100, valid, external
      100 54
        132.1.26.6 (metric 20) from 132.1.0.4 (150.1.4.4)
          Origin IGP, metric 0, localpref 100, valid, internal, best
          Originator: 150.1.2.2, Cluster list: 150.1.4.4

    after:

    Rack1R1(config-if)#do sh ip bgp 28.119.16.0
    output omitted   
      400 54 (with the 'no-prepend' command)
        132.1.17.7 from 132.1.17.7 (150.1.7.7)
          Origin IGP, localpref 100, valid, external, best
      100 54
        132.1.26.6 (metric 20) from 132.1.0.4 (150.1.4.4)
          Origin IGP, metric 0, localpref 100, valid, internal
          Originator: 150.1.2.2, Cluster list: 150.1.4.4

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