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CoS Device Configuration Overview

Before you begin configuring a Juniper Networks device for CoS, complete the following tasks:

  • Determine whether the device needs to support different traffic streams, such as voice or video. If so, CoS helps to make sure this traffic receives more than basic best-effort packet delivery service.

  • Determine whether the device is directly attached to any applications that send CoS-classified packets. If no sources are enabled for CoS, you must configure and apply rewrite rules on the interfaces facing the sources.

  • Determine whether the device must support assured forwarding (AF) classes. Assured forwarding usually requires random early detection (RED) drop profiles to be configured and applied.

  • Determine whether the device must support expedited forwarding (EF) classes with a policer. Policers require you to apply a burst size and bandwidth limit to the traffic flow, and set a consequence for packets that exceed these limits—usually a high loss priority, so that packets exceeding the policer limits are discarded first.


When the T1/E1 mPIM is oversubscribed, we recommend that you configure its shaping rate for consistent CoS functionality. The shaping rate should be less than the total link speed.