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BGP Problem with AS...pls help

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  • BGP Problem with AS...pls help

    Hello! In the image bellow you will see a topology that runs only BGP protocol everywhere, and it has two AS's. AS 9500 the core and AS 65002. The problem is that AS 65002 is split in 2 halfs and is connected togheter by AS 9500. The problem is that the loopback on R4 does not reach the loopback on R6...so they don't ping.
    How do i advertise the two halfs to each other in order to have connection and for the loopbacks to reach one another? I'll have to mention that i don't want to make any static routing or make the connection thru another dinamic protocol...i want to solve this by using only the BGP.
    So any ideas ???

    Thx in advance and here is the topology


  • #2
    Re: BGP Problem with AS...pls help

    Don't R2 and R3 have to participate in AS65002 in order to advertise those networks in order for traffic between R4 and R6 to reach each other?

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    • #3
      Re: BGP Problem with AS...pls help

      No, it cannot participate in AS 65002. i've used next-hop-self in R5 and R6 and they go out with internal routing for AS 65002 but they don't enter each other.
      I'm inclined to think that a possible solution would be with reflection router in BGP... what do you think... its a possible solution?

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      • #4
        Re: BGP Problem with AS...pls help

        I don't know anything about BGP. what I'm getting at is this:

        R5 and R6 know about each other and their networks through their membership in AS65002.

        R5 knows that to get to R6 it has to go through R2.

        R6 knows that to get to R5 it has to go through R3.

        R2 knows about R5 but not R6.

        R3 knows about R6 but not R5.

        So you have to make R2 know about R6 and you have to make R3 know about R5. Do I have it right?

        I was suggesting that R2 and R3 participate in AS65002 as a way to accomplish this. If you don't want to use another routing protocol then static routes would do the trick.

        Of course you may want to see if a BGP expert chimes in on this.

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        • #5
          Re: BGP Problem with AS...pls help

          you're allmost right... with static i have no doubt i'll achieve this, but the challenge i was put up to by a mentor is to do this and specifically said not thru static or dinamic...just thru bgp.
          As about the knowing part...well R2 knows of both R5 and R6 and again R3 the same... the loopbacks go into the core AS 9500 but they refuse to enter each other and that is the real problem...
          there maybe many possibilities...syncronization may be needed, of i'd have to mess with the TTL...

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          • #6
            Re: BGP Problem with AS...pls help

            Ok. sorry I couldn't provide any real help for you. Keep us posted on how you make out and hopefully someone here with BGP experience will lend their two cents.

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            • #7
              Re: BGP Problem with AS...pls help

              Route Reflectors are an iBGP concept so this would not work. As eBGP peers pass on all known iBGP and eBGP routes, you could say that they are always route reflectors in essence.

              The problem is that R6 will never receive routes from R5 becuase this would be seen as a routing loop. Loop avoidance is an important part of BGP so if a route is received which contains your own AS within the path, then this is seen as a routing loop and wont be accepted.

              If you look at the received routes on R5 then you might find that it is actually receiving the route from R6 but no installing it into the routing table because of this reason.

              Within AS-9500 try configuring a policy which only allows AS-9500 to contribute to the eBGP routes. Then both R6 and R5 should receive the relevant routes and only AS-9500 will be listed within the path so they will not see their own AS. This would be disabling the loop avoidance feature of BGP.
              Infrastructure Architect
              CCNA, CCNA Security, MCSE, JNCIS

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              • #8
                Re: BGP Problem with AS...pls help

                got it.... i used the command in (config-router):neighbor specific neigh allowas-in 5 and now it works fine.
                Thx all for the support

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                • #9
                  Re: BGP Problem with AS...pls help

                  Great news that its working.

                  If you have just used the "allowas-in" command then notice that you will see the same AS in the BGP table twice. This has disabled the loop detection feature of BGP allowing you to receive the route.

                  For a cleaner solution look at Confederations so that all routes are seen from the core AS.
                  Infrastructure Architect
                  CCNA, CCNA Security, MCSE, JNCIS

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