Packet classification refers to the examination of an incoming packet. This function associates the packet with a particular CoS servicing level. In the JUNOS software, classifiers associate incoming packets with a forwarding class and loss priority and, based on the associated forwarding class, assign packets to output queues. Two general types of classifiers are supported—behavior aggregate (BA) classifiers and multifield (MF) classifiers.
A behavior aggregate (BA) classifier operates on a packet as it enters the router. Using behavior aggregate classifiers the router aggregates different types of traffic into a single forwarding class to receive the same forwarding treatment. The CoS value in the packet header is the single field that determines the CoS settings applied to the packet. Behavior aggregate classifiers allow you to set the forwarding class and loss priority of a packet based on the Differentiated Services (DiffServ) code point (DSCP) value, DSCP IPv6 value, IP precedence value, MPLS EXP bits, or IEEE 802.1p value. The default classifier is based on the IP precedence value. For more information, see Default Behavior Aggregate Classifiers.
A multifield (MF) classifier is a second method for classifying traffic flows. Unlike the behavior aggregate classifier, a multifield classifier can examine multiple fields in the packet—for example, the source and destination address of the packet or the source and destination port numbers of the packet. With multifield classifiers, you set the forwarding class and loss priority of a packet based on firewall filter rules.