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Understanding How Forwarding Classes Assign Classes to Output Queues

This topic covers the following information:

Output Queue Assignments Based on Forwarding Class

It is helpful to think of forwarding classes as output queues. In effect, the end result of classification is the identification of an output queue for a particular packet.

CoS packet classification assigns an incoming packet to an output queue based on the packet’s forwarding class. Each packet is associated with one of the following default forwarding classes:

  • Expedited forwarding (EF)—Provides a low-loss, low-latency, low- jitter, 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 supports protocol control.

Devices That Support Up to Four Forwarding Classes

Some of the Juniper Networks routing platforms support up to four forwarding classes for classifying customer traffic. On these platforms, you can configure one of each type of default forwarding class. The following Juniper Networks routing platforms support up to four forwarding classes:

  • M7i Multiservice Edge Routers with Compact Forwarding Engine Boards (CFEBs)

  • M10i Multiservice Edge Routers with CFEBs


This list does not reference any Juniper Networks device that has reached its End of Life (EOL) period and its End of Support (EOS) milestone date.

Devices That Support Up to 16 Forwarding Classes

Other Juniper Networks routing platforms support up to 16 forwarding classes, which enables you to classify packets more granularly. For example, you can configure multiple classes of EF traffic: EF, EF1, and EF2. On these platforms, the Junos OS software supports up to eight output queues; therefore, if you configure more than eight forwarding classes, you must map multiple forwarding classes to single output queues. The following Juniper Networks routing and switching platforms support up to 16 forwarding classes and up to 8 output queues:

  • EX Series switches

  • M7i Multiservices Edge Routers with Enhanced Compact Forwarding Engine Boards (CFEB-Es)

  • M10i Multiservices Edge Routers with CFEB-Es

  • M120 Multiservices Edge Routers

  • M320 Multiservices Edge Routers

  • MX Series 5G Universal Routing Platforms

  • T Series Core Routers

  • PTX Packet Transport Routers

Default and Configurable Packet Loss Priority Values

By default, the loss priority is low. On most devices, you can configure high or low loss priority. On the following devices, you can configure high, low, medium-high, or medium-low loss priority:

  • M320 routers and T Series routers with Enhanced III Flexible PIC Concentrators (FPCs)

  • T640 routers with Enhanced Scaling FPC4s

  • PTX Series Packet Transport Routers

Configuration Statements Used to Configure and Apply Forwarding Classes

To configure CoS forwarding classes, include the forwarding-classes statement at the [edit class-of-service] hierarchy level: