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  • 1.  Question on MPLS load balancing

     
    Posted 10-29-2022 07:55

    Hi.

     

    I have the following LDP topology:

     

    R1 -- R2 -- R3 – R5

          |           |  

          + --- R4 -- +

     

    If configured a load balancing policy on R2 that results in load balancing of packets towards FEC R5.

     

    [edit]

    jcluser@P# run show route forwarding-table label 16 | find R2                                       

    Logical system: R2

    Routing table: default.mpls

    MPLS:

    Destination        Type RtRef Next hop           Type Index    NhRef Netif

    16                 user     0                    ulst  1048575     2

                                  30.30.30.2        Swap 16      902     2 lt-0/0/0.5

                                  20.20.20.2        Swap 16      903     2 lt-0/0/0.3

     

     

    What are the fields in the incoming MPLS packet that R2 looks at by default to decide on the next hop towards FEC R5?

     

    Thanks,

    Deepak

     


    Juniper Business Use Only



  • 2.  RE: Question on MPLS load balancing

    Posted 28 days ago
    R2 uses the received mpls top label  to do mpls lookup [ show route <R5> table mpls.0>]  . It seems both R4 and R3 are advertising a label toward R5.  Since you are deploying ldp , the 'route selection" for the ldp lsp  is based on IGP metric.   Likely both R4 and R3 show the same metric to R5, hence load balancing is in place.

    Regards,


  • 3.  RE: Question on MPLS load balancing

     
    Posted 19 days ago
    Hi cballadares.

    The question was when R2 is forwarding an MPLS packet along the LSP to R5, how will it decide whether to forward it to R3 or R4, since both are next hops for the same incoming MPLS label?

    --Deepak


  • 4.  RE: Question on MPLS load balancing

    Posted 18 days ago
    Hello,

    By default, when there are multiple equal-cost paths to the same destination for the active route, Junos OS uses a hash algorithm to choose one of the next-hop addresses to install in the forwarding table.

    Ones the forwarding table is populated , when we speak about mpls , the documentation says: "The selection of the actual forwarding next-hop is based on the hash computation result over select packet header fields and several internal fields such as interface index."  

    You can find the same statement, the implementation caveats , limitations and considerations at  https://www.juniper.net/documentation/us/en/software/junos/sampling-forwarding-monitoring/topics/concept/hash-computation-mpcs-understanding.html#id-understanding-the-algorithm-used-to-load-balance-traffic-on-mx-series-routers__d5485e746

    -cb.