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[ARCHIVE] QoS rewrite on EX-series switches for switched and routed packets

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Article ID: KB13381 KB Last Updated: 24 Aug 2017Version: 3.0
Summary:
There are differences in the way QoS rewrite takes place between EX-series switches running JUNOS 9.4 and prior versions. This document explains the differences, and provides an example for rewrite of switched frames using code prior to JUNOS 9.4; since the documentation only describes the procedure for rewriting routed packets.
Symptoms:
How do I rewrite QoS values (CoS, DSCP or IP-prec) on an EX?
Solution:
On an EX-series switch using JUNOS prior to version 9.4, a rewrite rule is applied only for routed packets be default. To apply the rewrite for switched frames, we need to apply a firewall filter on the egress VLAN or interface so that the rewrite rule may be applied. However in JUNOS 9.4 and after, rewrite-rules defined under class-of-service stanza apply to both routed and switched packets.

This document assumes that only QoS re-write is taking place on the EX, and there is no classification, queuing, or scheduling taking place.  Such a configuration was purposely omitted from this KB for simplicity and to focus on a working rewrite example. Please consult the EX JUNOS documentation for information on configuring QoS.

Example: Rewriting switched frames with DSCP 46 (EF) to IEEE CoS 6.

The frames are switched from ge-0/0/1 to ge-0/0/0. Using JUNOS  9.3R2.8 on EX, a traffic generator is used to pump IP packets with DSCP 46 in interface ge-0/0/1. Interface ge-0/0/0 can be connected to a PC which can receive tagged frames where a sniffer can show the incoming frames to verify if the remarking or rewriting took place.

EX configuration:

[edit]
root# show class-of-service
forwarding-classes {
    class voice queue-num 5;
}
rewrite-rules {
     ieee-802.1 voice {
         forwarding-class voice {
             loss-priority low code-point 110;
         }
     }
}

root# show firewall
family ethernet-switching {
     filter rewrite-filter {
         term voice {
             from {
                 dscp ef; # any condition could be specified here
             }
             then {
                 forwarding-class voice;
                 loss-priority low;
                 count voice-hits;
             }
         }
     }
}

root# show interfaces ge-0/0/0
unit 0 {
    family ethernet-switching {
        port-mode trunk;
        vlan {
            members 1;
        }
        filter {
            output rewrite-filter;
        }
    }
}


Note: The above filter could also be applied to VLAN 1 egress instead.

As seen above, the forwarding class (FC) needs to be defined else the configuration will not commit. The FC however is just a dummy mapped to queue 5 in this case, and it is not actually used since there is no classifier configuration.
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