CoS packet classification assigns an incoming packet to a forwarding-class that’s ultimately associated with an output queue. Each packet is associated with one of the following default forwarding classes:
- Expedited forwarding (EF)—Provides a low-loss, low-latency, low-jitter and assured bandwidth end-to-end service.
- Assured forwarding (AF)—Provides a group of values you can define and includes four subclasses: AF1, AF2, AF3, and AF4, each with three drop probabilities: low, medium and high.
- Best effort (BE)—Provides no service profile. For the best effort forwarding class, loss priority is typically not carried in a class-of-service (CoS) value and random early detection (RED) drop profiles are more aggressive.
- Network control (NC)—This class is typically high priority because it is used for the network controlling protocols (routing and network device management).
For more information, please see the following: https://www.juniper.net/techpubs/en_US/junos15.1/topics/concept/forwarding-classes-default-cos-config-guide.html