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Forwarding Classes

Forwarding classes allow you to group packets for transmission. Based on forwarding classes, you assign packets to output queues. The forwarding class plus the loss priority define the per-hop behavior (PHB in DiffServ) of a packet. J-series Services Routers and SRX-series services gateways 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.

By default, the SRX-series devices support 4 queues. You can use the following CLI statement to change that setting to eight queues:



[edit class-of-service]
chassis {


fpc x {
pic y {
max-queue-per-interface 8;
}
}
}

The new setting will take effect when the FPC is restarted.

Note: Queues 4 through 7 are not mapped to forwarding classes. To use queues 4 through 7, you must create custom forwarding class names and map them to the queues. For more information, see Forwarding Class Queue Assignments.

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

The following CLI commands can assign a forwarding class directly to packets received at a logical interface:



[edit class-of-service interfaces interface-name unit logical-unit-number]
forwarding-class class-name;

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