Configuring Rewrite Rules Based on PLP

date_range 29-Nov-23

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Rewrite rules take action on outgoing packets. When tricolor marking (TCM) is enabled, routers support four rewrite packet loss priority (PLP) designations: low, medium-low, medium-high, and high. To include the PLP for a rewrite rule, include the following statements at the [edit class-of-service] hierarchy level:

[edit class-of-service]
rewrite-rules {
    (dscp | dscp-ipv6 | exp | ieee-802.1 | inet-precedence) rewrite-name 					 {
        import (rewrite-name | default);
        forwarding-class class-name {
            loss-priority (low | medium-low | medium-high | high) code-point (alias | bits);
        }
    }
}

In Junos OS, rewrite rules only look at the forwarding class and packet loss priority of the packet (as assigned by a behavior aggregate or multifield classifier at ingress), not at the incoming CoS value, to determine the CoS value to write to the packet header at egress. The inputs for a rewrite rule are the forwarding class and the PLP. The output for a rewrite rule are the CoS values. In other words, a rewrite rule sets the CoS value for each packet exiting the interface with a specified forwarding class and PLP.

For example, if you configure the following, the 000000 CoS value is assigned to all packets exiting the interface with the assured-forwarding forwarding class and medium-high PLP:

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class-of-service {
    rewrite-rules {
        dscp dscp-rw {
            forwarding-class assured-forwarding {
                loss-priority medium-high code-point 000000;
            }
        }
    }
}

To use this classifier, you must configure the settings for the assured-forwarding forwarding class at the [edit class-of-service forwarding-classes queue queue-number assured-forwarding] hierarchy level. For more information, see Understanding How Forwarding Classes Assign Classes to Output Queues.

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Class of Service User Guide (Routers and EX9200 Switches)

Configuring Rewrite Rules Based on PLP

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