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Understanding CoS Forwarding Classes

 

Class-of-Service (CoS) forwarding classes can be thought of as output queues. In effect, the result of classifying packets is the identification of an output queue for a particular packet. For a classifier to assign an output queue to a packet, it must associate the packet with one of the following forwarding classes:

  • best-effort (be)—Provides no service profile. Loss priority is typically not carried in a CoS value.

  • 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 two drop probabilities: low and high.

  • network-control (nc)—Supports protocol control and thus is typically high priority.

  • multicast best-effort (mcast-be)—Provides no service profile for multicast packets.

  • multicast expedited forwarding (mcast-ef)—Supports high-priority multicast packets.

  • multicast assured-forwarding (mcast-af)—Provides two drop profiles; high, and low, for multicast packets.

  • multicast network-control (mcast-nc)—Supports high-priority multicast packets that are not delay-sensitive.

Note

The forwarding classes multicast expedited-forwarding, multicast assured-forwarding, and multicast best-effort are applicable only to Juniper Networks EX8200 and EX4300 Ethernet Switches. The forwarding class multicast network-control is applicable only to EX4300 switches.

Juniper Networks EX Series Ethernet Switches support up to 16 forwarding classes, thus allowing granular packet classification. For example, you can configure multiple classes of expedited forwarding (EF) traffic such as EF, EF1, and EF2.

EX Series switches support up to eight output queues, except EX4300 switches that support 12 output queues. Therefore, if you configure more forwarding classes that the number of queues supported, you must map multiple forwarding classes to one or more output queues. On EX8200 Virtual Chassis, you can configure only eight forwarding classes and you can assign only one forwarding class to each output queue.

Note

On EX8200 Virtual Chassis, the queue number seven carries Virtual Chassis port (VCP) traffic and can also carry high-priority user traffic.

This topic describes:

Default Forwarding Classes

Table 1 shows the four default forwarding classes defined for unicast traffic, and Table 2 shows the default forwarding classes defined for multicast traffic.

Note

The default forwarding classes for multicast traffic are applicable only to EX8200 Virtual Chassis and EX4300 switches.

You can rename the forwarding classes associated with the queues supported on your switch. Assigning a new class name to an output queue does not alter the default classification or scheduling that is applicable to that queue. However, because CoS configurations can be quite complicated, we recommend that you avoid altering the default class names or queue number associations.

Table 1: Default Forwarding Classes for Unicast Traffic

Forwarding Class Name

Comments

best-effort (be)

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

expedited-forwarding (ef)

The software delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for packets in this service class. The software accepts excess traffic in this class, but in contrast to the assured forwarding class, the out-of-profile expedited-forwarding class packets can be forwarded out of sequence or dropped.

assured-forwarding (af)

The software 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 software accepts excess traffic, but it applies a tail drop profile to determine that excess packets are dropped, and not forwarded.

Two drop probabilities (low and high) are defined for this service class.

network-control (nc)

The software delivers packets in this service class with a high priority. (These packets are not delay-sensitive.)

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

Table 2: Default Forwarding Classes for Multicast Traffic on EX8200 Virtual Chassis and EX4300 Switches

Forwarding Class Name

Comments

multicast best-effort (mcast-be)

The software does not apply any special CoS handling to multicast packets. These packets are usually dropped under congested network conditions.

multicast expedited-forwarding (mcast-ef)

The software delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for multicast packets in this service class. The software accepts excess traffic in this class, but in contrast to the multicast assured forwarding class, out-of-profile multicast expedited-forwarding class packets can be forwarded out of sequence or dropped.

multicast assured-forwarding (mcast-af)

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

The software accepts excess traffic, but it applies a tail drop profile to determine if the excess packets are dropped and not forwarded.

Two drop probabilities (low and high) are defined for this service class.

multicast network-control (mcast-nc)

(EX4300 switches only) The software delivers packets in this service class with a high priority. (These packets are not delay-sensitive.)

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

The following rules govern queue assignment:

  • CoS configurations that specify more queues than the switch can support are not accepted. If you commit such a configuration, the commit fails and a message displays that states the number of queues available.

  • All default CoS configurations are based on queue number. The name of the forwarding class that is displayed in the default configuration for a queue number is that of the forwarding class currently associated with that queue.