Supported Platforms
Multifield Classifier Overview
A multifield classifier is a method of classifying traffic flows. Devices that sit at the edge of a network usually classify packets according to codings that are located in multiple packet header fields. Multifield classification is normally performed at the network edge because of the general lack of DiffServ code point (DSCP) or IP precedence support in end-user applications.
In an edge router, a multifield classifier provides the filtering functionality that scans through a variety of packet fields to determine the forwarding class for a packet. Typically, a classifier performs matching operations on the selected fields against a configured value.
Unlike a behavior aggregate (BA), which classifies packets based on class-of-service (CoS) bits in the packet header, a multifield classifier can examine multiple fields in the packet header—for example, the source and destination address of the packet, and the source and destination port numbers of the packet. A multifield classifier typically matches one or more of the six packet header fields: destination address, source address, IP protocol, source port, destination port, and DSCP. Multifield classifiers are used when a simple BA classifier is insufficient to classify a packet.
In the Juniper Networks® Junos® operating system (Junos OS), you configure a multifield classifier with a firewall filter and its associated match conditions. This enables you to use any filter match criteria to locate packets that require classification. From a CoS perspective, multifield classifiers (or firewall filter rules) provide the following services:
- Classify packets 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.
- Police traffic to a specific bandwidth and burst size. Packets exceeding the policer limits can be discarded, or can be assigned to a different forwarding class, to a different loss priority, or to both.
 | Note: You police traffic on input to conform to established CoS parameters, setting loss handling and forwarding class assignments as needed. You shape traffic on output to make sure that router resources, especially bandwidth, are distributed fairly. However, input policing and output shaping are two different CoS processes, each with their own configuration statements. |
