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Is it possible to break iBGP default advertisement rule (do not tell iBGP learned route to other iBGP neighbours) without route reflector or confederations?

  • 1.  Is it possible to break iBGP default advertisement rule (do not tell iBGP learned route to other iBGP neighbours) without route reflector or confederations?

    Posted 11-28-2017 14:56

    Hello dear collegues,

     

    Since I do not have a lab to test these two scenarios, i would like to discuss these two questions with you here in case someone has tested it.

     

    We know that by default iBGP will not tell iBGP routes to other iBGP peers so IBGP  requires full mesh.

    We can use route reflectors or confederations to override this.

    My questions on this is: Imagine this scenario A B and C routers talk to Ibgp to each other

    ibgp between B and A

    ibgp between A and C

    B <------> A<------>C

    Can we explicitly configure "export policy" on one  router A,   to match other iBGP learned routes from B and advertise them to router C?

    Will router C accept these routes?

    If no, can we configure explicit import policy on router C and accept these routes?

    if yes, can we match based on communities?

    In short, can we break iBGP rules by simply using export import rules?

     

    My second question is more like to confirm,

    MED: If route has no MED and we do not overide it while we advertise it to other peer by default MED value is assumed to be 0 (zero)

    In this case on the other side if I receive the two the same prefixes with all atributes equal (LP, AS path, Origin) one has MED 10 other has no MED.

    Which one will be preferred? 

    According to cisco it will honour path with no MED. ( 0 med is the strongest)

    Is it the same for Junos?

     

    Thanks in advance!

    I would love to test these in lab, but do not have one yet.

     

     



  • 2.  RE: Is it possible to break iBGP default advertisement rule (do not tell iBGP learned route to other iBGP neighbours) without route reflector or confederations?

     
    Posted 11-28-2017 21:07

     

    @HasanovElshan wrote:

     

    In short, can we break iBGP rules by simply using export import rules?

     

     

     

    This is not possible, you can always peer B with C to create a full mesh 🙂

     

    @HasanovElshan wrote:

     


    According to cisco it will honour path with no MED. ( 0 med is the strongest)

    Is it the same for Junos?

     

     The absence of a MED value is interpreted as a MED of 0 which is preferred, note that by default MED is only compared between routes coming from the same peer AS.

     

    Best regards.



  • 3.  RE: Is it possible to break iBGP default advertisement rule (do not tell iBGP learned route to other iBGP neighbours) without route reflector or confederations?
    Best Answer

    Posted 11-30-2017 05:09

    ibgp between A and C

    B <------> A<------>C

    Can we explicitly configure "export policy" on one  router A,   to match other iBGP learned routes from B and advertise them to router C?

     

    No, for iBGP those routes are not going to be sent to the other peer.  There are two options in that architecture:

    1-make A the cluster node of a route reflector with B & C as clients

    2-Implement as eBGP instead of iBGP