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IGP Loadbalancing

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  • 1.  IGP Loadbalancing

    Posted 06-26-2017 05:19
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    Hey Guys,


    Currently studying for the JNCIS-ENT and trying to understand loadbalancing.


    Just one aspect. The book says that the default IGP loadbalancing behaviour, is to only use one of the available paths for destinations. I keep re-reading this, but if I'm understanding correctly, it's saying that when two (or more) equal cost paths exist to the same destination, all traffic going to that destination will only flow down one of the available paths. If that's the case, how is this loadbalancing? Or have I misunderstood?


    Unless it's saying that, traffic destined to one destination will go down one path, traffic destined to another destination the next will go down the other and so on - in which case, how is that different to BGP's loadbalancing behaviour, assuming that "destination" is synonymous with "prefix"?


    The page that describes this has been attached.


    Thanks in advance,


    Kindest Regards,



  • 2.  RE: IGP Loadbalancing

    Posted 06-26-2017 05:28

    I think below words are the simple explanation:


    The default configuration in JunOS doesn’t have load-balancing enabled. Even if your IGP learned multiple equal-paths to the same destination, it shows you them in your routing-table, but only one next-hop is choosen to get installed into your forwarding-table.



  • 3.  RE: IGP Loadbalancing

    Posted 06-26-2017 05:30

  • 4.  RE: IGP Loadbalancing

    Posted 06-26-2017 06:23

    Hey man,


    I've already checked the last post you referenced before posting on this one. But it didn't really help understand.


    I know that by default load-balancing isn't enabled, however, when you do enable it, the difference between IGP and BGP's default behaviour is somewhat unclear.


    I will check out the JNCIE hints page you posted, looks tasty.


    Thanks 🙂 


    Kindest Regards,



  • 5.  RE: IGP Loadbalancing
    Best Answer

    Posted 06-26-2017 06:33

    Lol FFS,


    It seems that I already understand the concept, I just didn't understand how the book was laid out.


    The book isn't saying that when you enable loadbalancing, the default behaviour of an IGP is to select one route, it's saying before you enable loadbalancing the default is to use a single path for a destination with equal cost paths.


    Sorry, I got confused.


    Thanks for the reply.


    Kindest Regards,


    Andrey Leo

  • 6.  RE: IGP Loadbalancing

    Posted 06-26-2017 21:59

    Hi Folks,
    I hope the below content is simple and plain!


    BGP load-sharing
    BGP multipath (same destination prefix points to more than one physical path) can be configured for both EBGP and IBGP peers


    When all attributes are equal (except for peer router-id and peer ip address) a forwarding-table policy must be configured to turn on equal-cost multipath (ECMP):


    set routing-options forwarding-table export juniper-lb
    set policy-options policy-statement juniper-lb term lb-prod-prefixes then load-balance per-packet