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Cisco LAN Switching Fundamentals by Basir Sakandar - CCIE No. 6040, David Barnes - CCIE No. 6563

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Class of Service

QoS implementation can occur at both Layer 2 and Layer 3, and also work in conjunction to provide end-to-end QoS for services on the network. QoS implementation at Layer 2 is known as class of service (CoS). A Layer 2 device, such as a Catalyst switch, will provide preferential treatment for traffic that has higher Layer 2 CoS settings.

Packets are given priority based on their CoS value. The CoS values range from 0 to 7, with 7 as the highest priority level. The CoS resides inside the IEEE 802.1Q and Inter-Switch Link (ISL) headers. The three most significant bits in the 4-byte Tag field in IEEE 802.1Q are used for CoS. In the ISL header, the three least significant bits in the 1-byte User field carry the CoS value. Chapter ...

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