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Forum Replies

  1. Hi Jan,

    Before I try to answer your question, I guess you want to be dual-homed so that you have a backup path but you want to be sure that your DC is never used as a transit AS?

    There's a simple solution for this, you just need to make sure you don't advertise anything that you receive from ISP1 to ISP2 and the other way around. If that's what you are looking for then take a look here:

    http://networklessons.com/bgp/bgp-prevent-transit-as/

    If that's not what you want...let me know and I'll do my best to answer your question.

    Rene

  2. Hi Daniel,

    I might write a short tutorial on it with an example but it is a simple one. A BGP router will prefer routes that it installed into BGP itself over a route that another router installed in BGP.

    Rene

  3. Rene,

    Just trying to get this straight in my head. so path prepending is basically like adding hop count because it is a distance vector technology and it is a way of manipulating the direction along with a route map . do I have this right

    thank you for your great bgp lesson thus far. i'm at the bgp communities section

  4. Jon says:

    Hi @chriscowboyfann,

    Apologies for jumping into the thread here but I think you are absolutely correct in your analysis; this is a great way to think about path pre-pending. Different vendors implement it in different ways but the idea is the same. As a rule of thumb, we always try to control the BGP path selection using the easiest to understand and troubleshoot criteria available to us. Path length is a great parameter to manipulate if done carefully.

    When using AS path modification in production, you should be careful not to accidentally introduce backdoor routes so always test, check and re-check before and after implementation! :slight_smile:

    Kind regards,
    Jon

  5. Hello Chris

    BGP will choose the router ID based first on if the router-id command is present. It is not in this case. Secondly, it will choose the highest IPv4 IP address of a loopback interface. You are correct, that both R1 and R3 have the same loopback address (seen in the configs), and thus would be advertising the same router-id. So the next step would be to determine the lowest neighbour IP address, which is indeed 192.168.12.1. So you are correct, it does prefer 192.168.12.1 but not because of the router ID but because of the lowest neighbour IP address. I’ll inform @ReneMolenaar about the correction.

    Thanks!

    Laz

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