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Configuring CoS on EX Series Switches

The topics in this guide describe how to configure the Junos OS class-of-service (CoS) components. Junos CoS provides a flexible set of tools that enable you to fine tune control over the traffic on your network.

  • Define classifiers that classify incoming traffic into forwarding classes to place traffic in groups for transmission.

  • Map forwarding classes to output queues to define the type of traffic on each output queue.

  • Configure schedulers for each output queue to control the service level (priority, bandwidth characteristics) of each type of traffic.

  • Provide different service levels for the same forwarding classes on different interfaces.

  • Provide congestion management with tail drop profiles, queue shaping, and congestion notification.

  • Configure CoS on MPLS networks.

  • Configure various CoS components individually or in combination to define CoS services.


When you change the CoS configuration or when you deactivate and then reactivate the CoS configuration, the system experiences packet drops because the system momentarily blocks traffic to change the mapping of incoming traffic to input queues.

Table 1 lists the primary CoS configuration tasks, includes platform limitations, and provides links to those tasks.

Table 1: CoS Configuration Tasks

CoS Configuration Task


Basic CoS Configuration:

  • Configure CoS using EZQoS with templates for key traffic classes, or a browser and the J-Web interface.

  • Configure code-point aliases to assign a name to a pattern of code-point bits that you can use instead of the bit pattern when you configure CoS components such as classifiers and rewrite rules.

  • Configure classifiers and multidestination classifiers.

    • Configure rewrite rules to alter code-point bit values in outgoing packets on the outbound interfaces of a switch so that the CoS treatment matches the policies of a targeted peer.

    • Set the forwarding class and loss priority of a packet based on the incoming CoS value and assign packets to output queues based on the associated forwarding class.

  • Configure forwarding classes.

  • Configure priority-based flow control to apply link-level flow control on a specific traffic class so that different types of traffic can efficiently use the same network interface card (NIC).

  • Configure CoS schedulers to define the properties of output queues on EX Series switches. These properties include the amount of interface bandwidth assigned to the queue, the size of the memory buffer allocated for storing packets, the priority of the queue, and the drop profiles associated with the queue.

  • Assign the following CoS components to physical or logical interfaces:

    • Classifiers (logical interfaces only)

    • Forwarding classes (logical interfaces only)

    • Scheduler maps

    • Rewrite rules

Configure congestion management mechanisms for a switch to drop arriving packets based on certain parameters when a queue is full. Based on the EX Series switch that you are using, packets are dropped depending on the priority of a packet or on both priority and drop probability of a packet.

  • Configure a weighted tail drop profile, a simple and effective traffic congestion avoidance mechanism. When you apply this mechanism to manage congestion, packets are dropped when the output queue is full.

  • Configure a weighted random early detection (WRED) drop profile. When the configured capacity (fill level) is reached, packets marked with a packet loss priority (PLP) of high are discarded.

  • Configure port shaping and queue shaping to enable you to limit traffic on an interface or queue, respectively, so that you can control the amount of traffic passing through the interface or the queue.

  • Configure explicit congestion notification (ECN) to enable end-to-end congestion notification between two endpoints on TCP/IP based networks. Apply WRED drop profiles to forwarding classes to control how the switch marks ECN-capable packets.

Configure CoS on MPLS networks to ensure better performance for low-latency applications such as VoIP and other business-critical functions.