ebgp Load balancing

Hi All,

Can any one please brief me how EBGP load balancing will work by using command "maximum path" option.

/Ganpat

Comments

  • Hi Ganpat,

    BGP will add the multipath attribute and it will install x prefixes into the routing table and CEF will load-share, by default, it is per destination load-sharing.

    HTH

    Good luck!

  • BGP, by default, does not install multiple prefixes to the same destination into the RIB (routing table).  When you have more than one prefix, whose bgp attributes are the same, BGP will install the best one, or multiple if you use the maximum path option.

    The command option goes into the global BGP config.  You can select up to 6 prefixes.

     

    Reference:
    Cisco BGP-4 Command and Configuration Handbook, Pages 115-117
    Internet Routing Architectures, Pages 218-220 (By far my favorite BGP book)

     

     

  • Can any one please brief me how EBGP load balancing will work by using command "maximum path" option.

    The first and formost condition for bgp load balancing is that the bgp best path order up to the "IGP metric to the next hop" have to be same for the destination prefix. Then we can apply "max-path" under bgp process for load balancing as mentioned in other relies to this post.

  • Hi!

    Well there are two different ways to load share in bgp (at least I use them and know them)

     

    First one:

    Load-Share the underlying IGP. Imagine you have two routers that are connected with multiple links. What you can do here, assign the same IGP metric for those links (well when the links are the same) and let the session between the routers come up by the loopback interfaces of the routers. For BGP it is just one route to the other router, but for the igp its multiple routes. So the IGP wil load share the traffic.

     

    Second One:

    When you are connected to two ISPs then you can also load balance. Imagine you have a /24 subnet and you are primarily reachable through your first ISP and have ISP2 as backup. Thats somehow inefficent except you want it that way. Just split the /24 in two /25 networks and advertise one better over the first ISP and the other one over the second ISP. Thats the way your incoming traffic is shaped. Of course your outgoing traffic is usually sent out one link. But you can use policy routing for example to conquer that.

     

    HTH.

    Regards!

    Markus.

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