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Configuring Rewrite Rules Based on PLP

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:

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:

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.