Content Overview In any network where networked applications require differentiated levels of service, traffic must be sorted into different classes to which quality of service (QoS) is applied. Classification, marking, and queuing are critical functions of any successful QoS implementation. Classification allows network devices to identify traffic as belonging to a specific class with the specific QoS requirements determined by an administrative QoS policy. After network traffic is sorted, individual packets are marked so that other network devices can apply QoS features uniformly to those packets in compliance with the defined QoS policy. Queuing dispatches the packets according to their markings. This module introduces classification, marking, and queuing using various methods of implementation. Web Links Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.4
http://www.cisco.com/en/US/products/ps6350/products
_configuration_guide_book09186a0080435d50.html
This publication provides an excellent resource for reference and future study. It provides the theory, practical examples and solutions for every aspect of Cisco IOS QoS. DiffServ -- The Scalable End-to-End QoS Model
http://www.cisco.com/en/US/products/ps6610/products
_white_paper09186a00800a3e2f.shtml Branch QoS Design At-a-Glance
http://www.cisco.com/application/pdf/en/us/guest /tech/
tk759/c1482/cdccont_0900aecd80295aa8.pdf Network-Based Application Recognition and Distributed Network-Based Application Recognition
http://www.cisco.com/en/US/products/ps6350/products
_configuration_guide_chapter09186a0080455985.html Configuring NBAR
http://www.cisco.com/univercd/cc/td/doc/product/
software/ios122/122cgcr/fqos_c/fqcprt1/qcfnbar.htm Cisco AutoQoS White Paper
http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd8019f3e0.pdf Network Based Application Recognition Performance Analysis
http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd80295a9b.pdf Understanding Quality of Service on Catalyst 6000 Family Switches
 http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd80295ac3.pdf QoS Configuration and Monitoring Whitepapers
http://www.cisco.com/en/US/tech/tk543/tk759/
tech_white_papers_list.html
Content 4.1 Introducing Classification and Marking 4.1.1 Classification Classification is the process of identifying traffic and categorizing that traffic into classes. Classification uses a traffic descriptor to categorize a packet within a specific group to define that packet. Traffic descriptors that are typically used include: After the packet has been classified or identified, the packet is then accessible for QoS handling on the network. Using classification, network administrators can partition network traffic into multiple classes of service (CoS). When traffic descriptors are used to classify traffic, the source implicitly agrees to adhere to the contracted terms and the network promises QoS. Various QoS mechanisms, such as traffic policing, traffic shaping, and queuing techniques, use the traffic descriptor of the packet (that is, the classification of the packet) to ensure adherence to that agreement. Classification should take place at the network edge, typically in the wiring closet, within IP phones, or at network endpoints. Cisco recommends that classification occur as close to the source of the traffic as possible. Figure summarizes these points. Note
The term classification is interchangeable with the term packet classification.
Content 4.1 Introducing Classification and Marking 4.1.2 Marking Marking is related to classification. Marking allows network devices to classify a packet or frame at the edge based on a specific traffic descriptor. 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. Traffic descriptors that are typically used include these: Marking can be used to set information in the Layer 2 frame or Layer 3 packet headers. Marking a packet or frame with its classification allows subsequent network devices to easily distinguish the marked packet or frame as belonging to a specific class. After the packets or frames are identified as belonging to a specific class, QoS mechanisms can be uniformly applied to ensure compliance with administrative QoS policies. Figure summarizes these points.
Content 4.1 Introducing Classification and Marking 4.1.3 Classification and Marking at the Link Layer The 802.1Q standard is an IEEE specification for implementing VLANs in Layer 2 switched networks. The 802.1Q specification defines two 2-byte fields (tag protocol identifier [TPID] and tag control information