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Understanding CoS Default Settings

The Class of Service menu in J-Web allows you to configure most of the Junos OS CoS components for the IPv4 and MPLS traffic on a Juniper Networks device. You can configure forwarding classes for transmitting packets, define which packets are placed into each output queue, schedule the transmission service level for each queue, and manage congestion using a random early detection (RED) algorithm. After defining the CoS components, you must assign classifiers to the required physical and logical interfaces.

Even when you do not configure any CoS settings on your routing platform, the software performs some CoS functions to ensure that user traffic and protocol packets are forwarded with minimum delay when the network is experiencing congestion. Some default mappings are automatically applied to each logical interface that you configure. Other default mappings, such as explicit default classifiers and rewrite rules, are in operation only if you explicitly associate them with an interface.

You can display default CoS settings by running the show class-of-service operational mode command.

You configure CoS when you need to override the default packet forwarding behavior of a Juniper Networks device—especially in the three areas identified in Table 1.

Table 1: Reasons to Configure Class of Service (CoS)

Default Behavior to Override with CoS

CoS Configuration Area

Packet classification—By default, the Juniper Networks device does not use behavior aggregate (BA) classifiers to classify packets. Packet classification applies to incoming traffic.


Scheduling queues—By default, the Juniper Networks device has only two queues enabled. Scheduling queues apply to outgoing traffic.


Packet headers—By default, the Juniper Networks device does not rewrite CoS bits in packet headers. Rewriting packet headers applies to outgoing traffic.

Rewrite rules