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Notable Replies

  1. Rene,
    I thought multi-hop is only when we have two BGP neighbors are not directly connected. I can see in the example when loopback addresses are used to configure BGP neighbors you are using multi-hop command even thoug the routers are directly connected.
    In the first example and your note says having non BGP router between two BGP routers is not a good idea. For instance if we have no choice then how can we make sure R1 and R3 networks are reachable?
    Please clarify.


  2. Hi Hamood,

    That's right, if you source eBGP from the loopback interfaces then you'll need multihop.

    The problem with the R1-R2-R3 scenario is that R2 will never learn about any prefixes, when R1 tries to reach a network behind R3 (or R3 wants to reach something behind R1) then R2 will receive an IP packet with a destination it doesn't know about.

    If you really had to use eBGP between R1 and R3 then a GRE tunnel might be a solution...


  3. Venus,
    In this situation, you can do two things:

    1. (config-router)#neighbor ebgp-multihop 255

    2. Run a trace route from your router to the neighbor. Take the hop count from that. I usually pad it out by about three.

    Method #2 is the more desirable one, fyi.

  4. Hi Palani,

    Cisco IOS routers use CEF (Cisco Express Forwarding) for the forwarding of packets.

    The default is "per destination" load balancing but you can change it per interface. For example:

    R1(config)#interface GigabitEthernet 0/1
    R1(config-if)#ip load-sharing per-packet

    Routing protocols are used to learn different routes, CEF is used for the forwarding of traffic. If you change the load sharing type then it will apply to all traffic.


  5. andrew says:

    As the statement indicates, the default behavior of BGP is to choose only one best path. In order to do this, there is a somewhat complicated best path selection process - depending on how you count, it is 13 steps long! What the maximum-path command does is to tell BGP to stop at a certain point during that best path selection process, and basically say, "if you get this far in the selection process, then consider the paths equally good and use them both."

    In reality, it is rare for this feature actually to be used, because it stops the selection process pretty deep in the list. In order for multiple paths to be used, all of the following characteristics of the path must be equal:

    1. Weight
    2. Local Preference
    3. Local Origination Status
    4. AS Path Length
    5. Origin Code (i,e,?)
    6. MED (metric)
    7. Same IGP metric to exit points

    Here is a very detailed Cisco document that talks about BGP path selection, and how multi-path fits in:

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