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How do you set the EXP bits of an L3VPN's inner MPLS header?

  • 1.  How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 05:35

    Hi.

     

    In a basic L3VPN setup, I'm trying to configure an ingress PE to set the EXP bits of the inner MPLS header of traffic it sends to remote PEs.

     

    I'm trying to configure an IP firewall filter on the ingress (CE-facing) interface of the ingress PE, but I see not firewall action to set outgoing EXP bits (I only see an action to set DSCP bits).

     

    So I have two questions:

     

    1: Are the IP Precedence bits of the encapsulated IP header copied to the EXP bits of the inner MPLS header by default?

    2: How do I set the EXP bits of the inner MPLS header?

     

    I'm testing on a VMX.

     

    Many Thanks,

    Deepak

     


    Juniper Business Use Only



  • 2.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 05:54

    Hi, 

    EXP bits on MPLS header is set by CoS rewrite rules.
    I believe, there are some default EXP rewrite rules on junos which can be verified by:

    > show class-of-service rewrite-rule type exp


    Which exp code-points are written in the MPLS header depends on which Forwarding Class (FC) that traffic is in, which is determined by CoS classifiers.
    You would be able to set the FC using Firewall filter (MultiField Classifier) for a particular flow.

    Hope this helps.
    Cheers,



    ------------------------------
    Ashvin
    ------------------------------



  • 3.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 06:39
    Hi Ashvin.

    Thanks for the reply.

    The rewrite rule would set the EXP bits of the outer MPLS header,  but I'm interested in setting the EXP bits of the inner MPLS header.

    Thanks,
    Deepak


  • 4.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 07:18

    Hi, 

    Inner MPLS EXP marking is copied from outer MPLS EXP.
    Example below of pkt capture for L3VPN traffic:

    MultiProtocol Label Switching Header, Label: 262141, Exp: 5, S: 0, TTL: 64
        0011 1111 1111 1111 1101 .... .... .... = MPLS Label: 262141
        .... .... .... .... .... 101. .... .... = MPLS Experimental Bits: 5
        .... .... .... .... .... ...0 .... .... = MPLS Bottom Of Label Stack: 0
        .... .... .... .... .... .... 0100 0000 = MPLS TTL: 64
    MultiProtocol Label Switching Header, Label: 16, Exp: 5, S: 1, TTL: 64
        0000 0000 0000 0001 0000 .... .... .... = MPLS Label: 16
        .... .... .... .... .... 101. .... .... = MPLS Experimental Bits: 5
        .... .... .... .... .... ...1 .... .... = MPLS Bottom Of Label Stack: 1
        .... .... .... .... .... .... 0100 0000 = MPLS TTL: 64
    Internet Protocol Version 4, Src: 192.168.100.1, Dst: 192.168.102.10


    Cheers,



    ------------------------------
    Ashvin
    ------------------------------



  • 5.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 07:53
    Hi Ashvin.

    So there isn't away to have the EXP fields of both headers different? 

    Thanks,
    Deepak


  • 6.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

     
    Posted 07-15-2021 10:50
    Hi, 

    I am not aware of any method to have different EXP markings on inner and outer MPLS headers.
    However, in an explicit-null label design, the inner EXP marking is not relevant IMHO, as traffic will be classified based on the outer EXP.

    The inner EXP can be altered using rewrite rules at the penultimate hop.

    Curious about the use-case for this requirement.

    Cheers,

    ------------------------------
    Ashvin
    ------------------------------



  • 7.  RE: How do you set the EXP bits of an L3VPN's inner MPLS header?

    Posted 07-17-2021 08:48
    You would have to make the marking decision prior the packet going into the core, then all modifications for marking post label imposition would be made to the topmost label, and the inner label would have its own marking.

    Regards,

    Elvin