Marking

This topic describes the purpose of marking and identifies where marking is commonly implemented in a network.

Marking, also known as coloring, marks each packet as a member of a network class so that the packet class can be quickly recognized throughout the rest of the network.

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Marking, also known as coloring, involves marking each packet as a member of a network class so that devices throughout the rest of the network can quickly recognize the packet class. Marking is performed as close to the network edge as possible and is typically done using MQC.

QoS mechanisms set bits in the DSCP or IP Precedence fields of each IP packet according to the class that the packet is in. The settings for the DSCP field and their relationship to the IP Precedence fields are discussed in the "Understanding the Differentiated Services Model" lesson in this module. Other fields can also be marked to aid in the identification of a packet class.

Other QoS mechanisms use these bits to determine how to treat the packets when they arrive. If the packets are marked as high-priority voice packets, the packets will generally never be dropped by congestion avoidance mechanisms and will be given immediate preference by congestion management queuing mechanisms. On the other hand, if the packets are marked as low-priority file transfer packets, they will be dropped when congestion occurs and will generally be moved to the end of the congestion management queues.

Note The tools for marking are covered in detail in the "Classification and Marking" module in this course.

Congestion Management

This topic describes congestion management and identifies where congestion management is commonly implemented in a network.

Congestion Management

Congestion management uses the marking on each packet to determine in which queue to place packets.

Congestion management uses sophisticated queuing technologies, such as WFQ and LLQ, to ensure that time-sensitive packets such as voice are transmitted first.

Congestion management mechanisms (queuing algorithms) use the marking on each packet to determine in which queue to place packets. Different queues are given different treatment by the queuing algorithm based on the class of packets in the queue. Generally, queues with high-priority packets receive preferential treatment.

Congestion management is implemented on all output interfaces in a QoS-enabled network by using queuing mechanisms to manage the outflow of traffic. Each queuing algorithm was designed to solve a specific network traffic problem and has a particular effect on network performance.

The Cisco IOS software for congestion management or queuing include these queuing methods:

■ FIFO, priority queuing (PQ), custom queuing (CQ)

■ Weighted fair queuing (WFQ)

■ Class-based weighted fair queuing (CBWFQ)

LLQ is currently the preferred queuing method. LLQ is a hybrid (PQ and CBWFQ) queuing method that was developed to specifically meet the requirements of real-time traffic, such as voice.

Note All of the queuing technologies discussed are described further in the "Congestion

Management" module in this course.

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