Example: Writing Different DSCP and EXP Values in MPLS-Tagged IP Packets

On Juniper Networks M320 Multiservice Edge Routers and T Series Core Routers, you can selectively set the DSCP field of MPLS-tagged IPv4 and IPv6 packets to 000000. In the same packets, you can set the MPLS EXP field according to a configured rewrite table, which is based on the forwarding classes that you set in incoming packets using a BA or multifield classifier.

Queue selection is based on the forwarding classes you assign in scheduler maps. This means that you can direct traffic to a single output queue, regardless of whether the DSCP field is unchanged or rewritten to 000000. To do this, you must configure a multifield classifier that matches selected packets and modifies them with the dscp 0 action.

Selective marking of DSCP fields to 0, without affecting output queue assignment, can be useful. For example, suppose you need to use the MPLS EXP value to configure CoS applications for core provider routers. At the penultimate egress provider edge (PE) router where the MPLS labels are removed, the CoS bits need to be provided by another value, such as DSCP code points. This case illustrates why it is useful to mark both the DSCP and MPLS EXP fields in the packet. Furthermore, it is useful to be able to mark the two fields differently, because the CoS rules of the core provider router might differ from the CoS rules of the egress penultimate router. At egress, as always, you can use a rewrite table to rewrite the MPLS EXP values corresponding to the forwarding classes that you need to set.

For IPv4 traffic, the dscp 0 action modifier at the [edit firewall family inet filter filter-name term term-name then] hierarchy level is valid. However, for IPv6 traffic, you configure this feature by including the traffic-class 0 action modifier at the [edit firewall family inet6 filter filter-name term term-name then] hierarchy level.

In the following IPv4 example, term 1 of the multifield classifier matches packets with DSCP 001100 code points coming from a certain VRF, rewrites the bits to DSCP 000000, and sets the forwarding class to best-effort. In term 2, the classifier matches packets with DSCP 010110 code points and sets the forwarding class to best-effort. Because term 2 does not include the dscp 0 action modifier, the DSCP 010110 bits remain unchanged. Because the classifier sets the forwarding class for both code points to best-effort, both traffic types are directed to the same output queue.

[edit]
firewall {
    family inet {
        filter vrf-rewrite {
            term 1 {
                from {
                    dscp b001100;
                }
                then {
                    dscp 0;
                    forwarding-class best-effort;
                }
            }
            term 2 {
                from {
                    dscp b010110;
                }
                then {
                    forwarding-class best-effort;
                }
            }
        }
    }
}

Applying the Multifield Classifier

Apply the filter to an input interface corresponding to the VRF:

[edit]
interfaces {
    so-0/1/0 {
        unit 0 {
            family inet {
                filter input vrf-rewrite;
            }
        }
    }
}