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Forwarding Classes Overview

 

Forwarding classes (FCs) allow you to group packets for transmission and to assign packets to output queues. The forwarding class and the loss priority define the per-hop behavior (PHB in DiffServ) of a packet.

Juniper Networks devices support eight queues (0 through 7). For a classifier to assign an output queue (default queues 0 through 3) to each packet, it must associate the packet with one of the following 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 BE 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.

In addition to behavior aggregate (BA) and multifield (MF) classification, the forwarding class (FC) of a packet can be directly determined by the logical interface that receives the packet. The packet FC can be configured using CLI commands, and if configured, this FC overrides the FC from any BA classification that was previously configured on the logical interface.

The following CLI command can assign an FC directly to packets received at a logical interface:

This section contains the following topics:

Forwarding Class Queue Assignments

Juniper Networks devices have eight queues built into the hardware. By default, four queues are assigned to four FCs. Table 1 shows the four default FCs and queues that Juniper Networks classifiers assign to packets, based on the class-of-service (CoS) values in the arriving packet headers.

Note

Queues 4 through 7 have no default assignments to FCs and are not mapped. To use queues 4 through 7, you must create custom FC names and map them to the queues.

By default, all incoming packets, except the IP control packets, are assigned to the FC associated with queue 0. All IP control packets are assigned to the FC associated with queue 3.

Table 1: Default Forwarding Class Queue Assignments

Forwarding Queue

Forwarding Class

Forwarding Class Description

Queue 0

best-effort (BE)

The Juniper Networks device does not apply any special CoS handling to packets with 000000 in the DiffServ field, a backward compatibility feature. These packets are usually dropped under congested network conditions.

Queue 1

expedited-forwarding (EF)

The Juniper Networks device delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for packets in this service class.

Devices accept excess traffic in this class, but in contrast to assured forwarding, out-of-profile expedited-forwarding packets can be forwarded out of sequence or dropped.

Queue 2

assured-forwarding (AF)

The Juniper Networks device offers a high level of assurance that the packets are delivered as long as the packet flow from the customer stays within a certain service profile that you define.

The device accepts excess traffic, but applies a random early detection (RED) drop profile to determine whether the excess packets are dropped and not forwarded.

Three drop probabilities (low, medium, and high) are defined for this service class.

Queue 3

network-control (NC)

The Juniper Networks device delivers packets in this service class with a low priority. (These packets are not delay sensitive.)

Typically, these packets represent routing protocol hello or keepalive messages. Because loss of these packets jeopardizes proper network operation, delay is preferable to discard.

Forwarding Policy Options

CoS-based forwarding (CBF) enables you to control next-hop selection based on a packet’s CoS and, in particular, the value of the IP packet's precedence bits. For example, you can specify a particular interface or next hop to carry high-priority traffic while all best-effort traffic takes some other path. CBF allows path selection based on FC. When a routing protocol discovers equal-cost paths, it can either pick a path at random or load-balance the packets across the paths, through either hash selection or round-robin selection.

A forwarding policy also allows you to create CoS classification overrides. You can override the incoming CoS classification and assign the packets to an FC based on the packets’ input interfaces, input precedence bits, or destination addresses. When you override the classification of incoming packets, any mappings you configured for associated precedence bits or incoming interfaces to output transmission queues are ignored.