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Forum Replies

  1. Hi Hamood,

    Good question, there’s a good explanation for this:

    1. iBGP requires a full mesh of peerings because of iBGP split horizon. This is why we use loopback interfaces instead of physical interfaces for the peering. Physical interfaces can go down, loopbacks can’t (unless you shut them). In this example I could have used physical interfaces since there is only one link between R2-R3 and R3-R4, if we had a link between R2-R4 then it would have been a must.

    2. When R4 (or R2) advertises its network on the loopback interface to R3 then R3 will store it in its BGP table but will never forward this network to R2 (or R4) because of iBGP split horizon. It will be impossible to establish iBGP between R2-R4 using loopback interfaces so you are forced to use physical interfaces.

    3. using an IGP (like OSPF) ensures that all iBGP routers can learn about the networks on the loopback interfaces. Also, when a physical link goes down…our IGPs are really fast switching to a backup route, BGP is slow…

    Hope this helps, if you need some more detail just let me know ok?


  2. Hello Rene,
    in the example above i got confused with the next-self-hop command. As we see that R4 have not learned the network!!! i wonder why R4 did not learn it from IGP/iBGP? my question is does IGP carry and advertise iBGP routes? can you please explain the benefit of the next-self-hop command ??? Thanks


  3. Technically, you can advertise whatever you want in BGP but in reality, it’s not going to happen.

    Let’s say you are connected to ISP1 and ISP2 and connected through BGP to learn some prefixes from them. If you don’t have any address space of your own, they will probably not accept anything that you do try to advertise by filtering everything inbound. If not, you could end up becoming a transit AS since you might advertise address space from ISP1 to ISP2 and vice versa.

    Even if they do accept some prefixes that you advertise, you would be advertising address space that doesn’t belong to you. If you want a good example, here’s how Pakistan knocked Youtube of the Internet :wink:

  4. Thanks for the long discussion, Rene.

    Yeap ! I will keep doing simple labs, until I get a better understanding of adavanced stuff, like BGP FREE CORE, SPLIT HORIZON, MPLS VPN, … :smile:

  5. Thanks! I just fixed it.

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