Before You Begin

Before you begin configuring a J–series or SRX-series device for CoS, complete the following tasks:

• If you do not already have a basic understanding of CoS, read Class-of-Service Overview.
• Determine whether the J–series or SRX-series 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 J–series or SRX-series 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 J–series or SRX-series 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 J–series or SRX-series 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.