Chapter 4. Classification and Marking

QOS Exam Topics

This chapter covers the following exam topics specific to the QoS exam:

• Explain how link layer and network layer markings are used to define service classes and the different applications represented by each of these service classes

• Given a network and a description of QoS issues, use MQC CLI commands to classify packets

• Given a network and a description of QoS issues, use class-based marking to assign packets to a specific service class

• Describe the function of Network Based Application Recognition

• Describe the purpose of pre-classification to support QoS in various VPN (IPSEC, GRE, L2TP) configurations

• Describe QoS trust boundaries and their significance in LAN based classification and marking

QoS classification tools categorize packets by examining the contents of the frame, cell, and packet headers, whereas marking tools allow the QoS tool to change the packet headers for easier classification. Many QoS tools rely on a classification function to determine to which traffic the tool applies. To place voice and data traffic in separate queues, for example, you must use some form of classification to differentiate the two types of traffic and place the identified traffic in the proper queue. Marking provides a way for QoS tools to change bits in the packet header to indicate the level of service this packet should receive from other QoS tools. For instance, you can use marking tools to change the marking in voice packets to ensure that a classification tool can differentiate a voice packet from a data packet. Without the marking feature, the frame, packet, or cell remains unchanged.

Marking involves placing a value into one of the small number of well-defined frame, packet, or cell header fields specifically designed for QoS marking. By marking a packet, other QoS functions can perform classification based on the marked field inside a header. Marking simplifies the network's QoS design, it simplifies configuration of other QoS tools, and it reduces the overhead required by each of the other QoS tools to classify the packets.

Although classification and marking tools do not directly affect the bandwidth, delay, jitter, or loss experienced by traffic in the network, classification and marking tools are the building blocks for all other QoS tools. With these tools, all traffic on the network is identified for the next QoS tool to act upon.

The concepts that apply to all classification and marking are covered in the first section of this chapter, including the terminology, fields used, and the meaning behind each of the available marked fields. Following that, this chapter covers several classification and marking tools, with example configurations, as well as show, and debug commands.