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Understanding How Behavior Aggregate Classifiers Prioritize Trusted Traffic

The idea behind class of service (CoS) is that packets are not treated identically by the

routers or switches on the network. In order to selectively apply service classes to specific

packets, the packets of interest must be classified in some fashion.

The simplest way to classify a packet is to use behavior aggregate (BA) classification,

also called the CoS value in this document. The DSCP, DSCP IPv6, or IP precedence bits

of the IP header convey the behavior aggregate class information. The information might

also be found in the MPLS EXP bits, IEEE 802.1ad, or IEEE 802.1p CoS bits.

NOTE: Support was added for filtering on Differentiated Services Code Point

(DSCP) and forwarding class for Routing Engine sourced packets, including

IS-IS packets encapsulated in generic routing encapsulation (GRE).

Subsequently, when upgrading from a previous version of Junos OS where

you have both a class of service (CoS) and firewall filter, and both include

DSCP or forwarding class filter actions, the criteria in the firewall filter

automatically takes precedence overthe CoS settings. The same is true when

creating new configurations; that is, where the same settings exist, the firewall

filter takes precedence over the CoS, regardless of which was created first.

BA classification is useful if the traffic comes from a trusted source and the CoS value

in the packet header is trusted. If the traffic is untrusted, multifield classifiers (see

“Overview of Assigning Service Levels to Packets Based on Multiple Packet Header Fields”

on page 891) are used to classify packets based on multiple packet fields. It is common

to use multifield classifiers to classify traffic at the ingress of a network, rewrite the packet

headers (see Rewriting Packet Headers to Ensure Forwarding Behavior), then use the more

efficient BA classification for transversing the network.

The BA classifier maps a CoS value in the packet header to a forwarding class and loss

priority. The forwarding class determines the output queue. The loss priority is used by

schedulers in conjunction with the random early discard (RED) algorithm to control

packet discard during periods of congestion.

Figure 59 on page 950 provides a high-level illustration of how a classifier works.

Figure 59: How a Classifier Works

ACX Series Universal Access Router Configuration Guide

Juniper ACX2000 Configuration Guide

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