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Forum Replies

  1. Hi Georgi,

    When you change the next hop on R2 then R1 will know how to reach R3.

    Once R3 receives the packet, does it know how to reach the destination? ( It probably doesn’t unless you advertise that network on R2.

    When you face problems like this, you need to “follow” the IP packet. Think about the source/destination IP address in the packet and then check if each router knows how to forward it :slight_smile:


  2. Nicholas,
    The reason for this is why BGP is different from other routing protocols. In fact, it is legitimate to view BGP not as a routing protocol at all, but instead as an application that supplies you with what’s called NLRI (Network Layer Reachability Information). Think of it this way: While routing protocols tell you HOW to get somewhere (they are always concerned with the next hop on a hop-by-hop basis), BGP tells you WHERE you need to go, but not how. It is up to the router to figure out how to get to the destination supplied by BGP, by looking at its own routing table / CEF to figure out the next hop.

    With this BGP philosophy in mind, by not changing to Next Hop Self, BGP allows your internal routing protocols to make the determination what is the best exit point in your internal network to get to the Next Hop being advertised by BGP. Of course, this assumes that your IGP knows about the link in question.

    As further evidence of this philosophy, look at the Administrative Distance of internal BGP: 200! Why is it so high? It is because the designers of BGP want iBGP to be the last resort (compared to any true internal routing protocol) for internal routing purposes.

  3. Hi Hoan,

    iBGP uses split horizon which means that it won’t advertise prefixes to iBGP neighbors that is has learned from other iBGP neighbors. You can read more about it here:

    Internal BGP explained


  4. Hello Brian

    It’s nice to see how you are working through the problem and we can “read your thoughts” along the way.

    So, having answered your own original question, here’s the question that I’ll try to answer:

    You must remember one of the fundamental laws of routing: If a route is successful in one direction, it does NOT mean that it will be successful in the other. So R3 was able to communicate with R2 and R1 the route to the network. So it is possible for R1 to reach R3. This is the perspective or R1 since there is a route from R1 to However, if you looked at the routing table of R3 (which you did) you will see that there is no route back to R1 without the appropriate network commands.

    So looking at the routing table in R1 only shows you that R1 can reach R3. It says nothing about R3 reaching R1. In order to determine that you’ll have to look at the routing table on R3.

    I hope this has been helpful!


  5. OMG!

    So I can apply that rule to routes? Meaning as long as it can get there it will be added just not reachable because it knows how to get there but not the way back.

    That does make perfect sense…gosh… I should have thought about that or at least posed it as a question. I was thinking that it probably had to do with some rule I had learned at some point about TCP/IP and traffic movement but just didn’t think of what was right in front of me. I hate when I do that but that happens a lot lol…

    Ok now that makes sense and I can put it in a nice little box and label it lol… thanks!

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