6 Replies Latest reply: Apr 1, 2014 10:12 PM by manoj RSS

    Confused in understanding bgp local-as no-prepend and replace-as ?

    manoj

      yesterday i labbed up this topic,i understood what this command can do.

       

       

      but i am confused regarding it's working

       

       

      my understanding is when we peer to another peer using local-as we are sending both the real-as and

      local-as in the bgp OPEN message and in our updates.

       

       

      say there are 3 routers like this

       

       

       

       

      r1------------>r2-------------------->r3

       

       

      Assume r1 and r2 are peering(ebgp peers) with local-as and r2 and r3 are peering with real-as.

       

       

       

       

      Assume every router has a loopback(with x.x.x.x x=router no) network published in their bgp process.

       

       

       

       

      1)what i observer was,when r2 receives a update from r1 for 1.1.1.0 network it contains the local-as

      number of r2.i am confused here,because as per our loop prevention mechanism we should ignore the

      update which contain our own AS number isn't it ?

       

       

      2)Also the purpose of "neighbor <ip> local-as <no> no-prepend replace-aS" is to keep the local peering as no with the local-peers,

      and keep the real-as number only visible between the ebgp peers with REAL AS (i.e mentioned in router bgp x)

      right ?

       

       

      my question is what is the reason to send both the AS-numbers in our open message and updates instead of sending

      only the respective AS no in the bgp updates and open message on a per neighbor basis ?

       

       

      A bit of clarification needed