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Configuring Basic Packet Flow Through the Junos OS CoS Process

Figure 1 and Figure 2 show the components of the Junos OS CoS features, illustrating the sequence in which they interact.

Figure 1: CoS Classifier, Queues, and SchedulerCoS Classifier, Queues, and Scheduler
Figure 2: Packet Flow Through CoS- Configurable ComponentsPacket Flow Through CoS- Configurable Components

The following configuration demonstrates the packet flow through the CoS process:

Define Classifiers

If you trust the CoS values in the packet headers, you can use behavior aggregate classification to map those values to a forwarding class and drop priority. For example:

If you do not trust the CoS values in the packet headers, you can use the more complex multifield classification to map ingress traffic to a forwarding class and drop priority. For example:

Apply Classifiers to Incoming Packets on Interfaces

You apply behavior aggregate classifiers to logical interfaces at the [edit class-of-service interfaces] hierarchy level. For example:

You apply multifield classifiers as input filters to logical interfaces at the [edit interfaces] hierarchy level. For example:

Define Policers to Limit Traffic and Control Congestion

If you need to rate-limit a traffic flow, either by discarding excess traffic (hard policing) or reassign excess traffic to a different forwarding class and/or loss priority (soft policing), define a policier and apply the policer to a firewall filter for that traffic flow. For example:

Define Drop Profiles

Use drop profiles to define the drop probabilities across the range of delay-buffer occupancy, supporting the random early detection (RED) process.

Assign Each Forwarding Class to a Queue

To provide differentiated services to each forwarding class, assign each forwarding class to it’s own output queue. For example:

Define Schedulers

Define the scheduler characteristics for each forwarding class. For example:

Define Scheduler Maps

Use scheduler maps to map schedulers to forwarding classes. For example:

Define CoS Header Rewrite Rules

Use rewrite rules to redefine the CoS bit pattern of outgoing packets. For example:

Apply Scheduler Maps and Rewrite Rules to Egress Interfaces

Release History Table
Starting with Junos OS Release 21.2, Junos OS displays class of service configuration in alphabetical order regardless of configuration order.