Table Of Contents
Classifying network traffic allows you to organize traffic (that is, packets) into traffic classes or categories on the basis of whether the traffic matches specific criteria. Classifying network traffic (used in conjunction with marking network traffic) is the foundation for enabling many quality of service (QoS) features on your network.
Packet classification is pivotal to policy techniques that select packets traversing a network element or a particular interface for different types of QoS service. For example, you can use classification to mark certain packets for IP Precedence, and you can identify other packets as belonging to a Resource Reservation Protocol (RSVP) flow.
Methods of classification were once limited to use of the contents of the packet header. Current methods of marking a packet with its classification allow you to set information in the Layer 2, 3, or 4 headers, or even by setting information within the payload of a packet. Criteria for classification of a group might be as broad as "traffic destined for subnetwork X" or as narrow as a single flow. For more information about classifying network traffic, see the "Classifying Network Traffic" chapter.
This chapter explains IP Precedence, and then it gives a brief description of the kinds of traffic classification provided by the Cisco IOS QoS features. It discusses features described in the following sections:
About IP Precedence
Use of IP Precedence allows you to specify the class of service (CoS) for a packet. You use the three precedence bits in the type of service (ToS) field of the IP version 4 (IPv4) header for this purpose. Figure 1 shows the ToS field.
Figure 1 IPv4 Packet Type of Service Field
Using the ToS bits, you can define up to six classes of service. Other features configured throughout the network can then use these bits to determine how to treat the packet. These other QoS features can assign appropriate traffic-handling policies including congestion management strategy and bandwidth allocation. For example, although IP Precedence is not a queueing method, queueing methods such as weighted fair queueing (WFQ) and Weighted Random Early Detection (WRED) can use the IP Precedence setting of the packet to prioritize traffic.
By setting precedence levels on incoming traffic and using them in combination with the Cisco IOS QoS queueing features, you can create differentiated service. You can use features such as policy-based routing (PBR) and committed access rate (CAR) to set precedence based on extended access list classification. These features afford considerable flexibility for precedence assignment. For example, you can assign precedence based on application or user, or by destination and source subnetwork.
So that each subsequent network element can provide service based on the determined policy, IP Precedence is usually deployed as close to the edge of the network or the administrative domain as possible. You can think of IP Precedence as an edge function that allows core, or backbone, QoS features such as WRED to forward traffic based on CoS. IP Precedence can also be set in the host or network client, but this setting can be overridden by policy within the network.
The following QoS features can use the IP Precedence field to determine how traffic is treated:
•Distributed WRED (DWRED)
How the IP Precedence Bits Are Used to Classify Packets
You use the three IP Precedence bits in the ToS field of the IP header to specify CoS assignment for each packet. You can partition traffic into a maximum of six classes and then use policy maps and extended access lists to define network policies for congestion handling and bandwidth allocation for each class.
For historical reasons, each precedence corresponds to a name. These names, which continue to evolve, are defined in RFC 791. Table 1 lists the numbers and their corresponding names, from least to most important.
Table 1 IP Precedence Values
However, the IP Precedence feature allows you considerable flexibility for precedence assignment. That is, you can define your own classification mechanism. For example, you might want to assign precedence based on application or access router.
Note IP Precedence bit settings 6 and 7 are reserved for network control information such as routing updates.
Setting or Changing the IP Precedence Value
By default, the Cisco IOS software leaves the IP Precedence value untouched, preserving the precedence value set in the header, allowing all internal network devices to provide service based on the IP Precedence setting. This policy follows the standard approach that stipulates that network traffic should be sorted into various types of service at the basic perimeter of the network and that those types of service should be implemented in the core of the network. Routers in the core of the network can then use the precedence bits, for example, to determine the order of transmission, the likelihood of packet drop, and so on.
Because traffic that enters your network can have precedence set by outside devices, we recommend that you reset the precedence for all traffic that enters your network. By controlling IP Precedence settings, you prohibit users that have already set the IP Precedence from acquiring better service for their traffic simply by setting a high precedence for all of their packets.
You can use CAR to set the IP Precedence in packets. As mentioned previously, after a packet has been classified, you can use other QoS features such as CAR and WRED to specify and enforce business policies to fit your business model.
Committed Access Rate
CAR is a multifaceted feature that implements both classification services and policing through rate limiting. This section describes the classification services of CAR. For information on CAR's rate limiting features, see the "Policing and Shaping Overview" chapter.
Note In Cisco IOS Release 12.2 SR, the classification services of CAR are not supported on the Cisco 7600 series router.
You can use the classification services of CAR to set the IP Precedence for packets that enter the network. This capability of CAR allows you to partition your network into multiple priority levels or classes of service. Networking devices within your network can then use the adjusted IP Precedence to determine how to treat the traffic. For example, VIP-distributed WRED uses the IP Precedence to determine the probability a packet being dropped.
As discussed in the "About IP Precedence" section, you can use the three precedence bits in the ToS field of the IP header to define up to six classes of service.
You can classify packets using policies based on physical port, source or destination IP or MAC address, application port, IP protocol type, or other criteria specifiable by access lists or extended access lists. You can classify packets by categories external to the network, for example, by a customer. After a packet has been classified, a network can either accept or override and reclassify the packet according to a specified policy. CAR includes commands that you can use to classify and reclassify packets.
CAR is supported on the majority of Cisco routers. Additionally, distributed CAR is supported on Cisco 7000 series routers with an RSP7000 interface processor or Cisco 7500 series routers with a VIP-based VIP2-40 or greater interface processor. A VIP2-50 interface processor is strongly recommended when the aggregate line rate of the port adapters on the VIP is greater than DS3. A VIP2-50 interface processor is required for OC-3 rates.
For information on how to configure CAR, see the "Configuring Committed Access Rate" chapter.
Classifying Network Traffic Using NBAR
Network-Based Application Recognition (NBAR) is a classification engine that recognizes and classifies a wide variety of protocols and applications. When NBAR recognizes and classifies a protocol or application, the network can be configured to apply the appropriate QoS for that application or traffic with that protocol.
For more information about NBAR, see the "Classifying Network Traffic Using NBAR" chapter.
Marking Network Traffic
Marking network traffic allows you to set or modify the attributes for traffic (that is, packets) belonging to a specific class or category. When used in conjunction with network traffic classification, network traffic marking is the foundation for enabling many QoS features on your network.
For more information about marking network traffic, see the "Marking Network Traffic" chapter.
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