Intel 460T manual Tagged Frames, Priority

C H A P T E R 2 Using the Intel Express 460T Standalone Switch

Tagged Frames

The 802.1D (1998 Edition) and 802.1Q specifications published by the IEEE (Institute of Electrical and Electronic Engineers) extended Ethernet functionality to add tag information to Ethernet frames and propagate these tagged frames between bridges (for example, a switch). The tag can carry priority information, VLAN information, or both and enables bridges to intelligently direct traffic across the network.

Priority

The IEEE 802.1D (1998 Edition) specification incorporates IEEE 802.1p and defines information in the frame tag to indicate a priority level. When these tagged packets are sent out on the network, the higher priority packets are transferred first. Priority packet tagging (also known as Traffic Class Expediting) is usually set on the LAN adapter in a PC and works with other elements of the network (switches, routers) to deliver priority packets first. The priority level can range from 0 (low) to 7 (high).

The 460T switch can read the priority tags and forward traffic on a per port basis. The switch uses two priority queues per port and routes traffic to a queue depending on the packet’s tag. For example, when a packet comes into the switch with a high-priority tag, the switch routes the packet to its high-priority queue.

Although there are eight priority levels, the 460T switch can only route a packet into one of the two queues. The switch maps levels 0-3 to the low queue (which is the default) and levels 4-7 to the high queue. If a packet is untagged, the switch determines the best way to send the packet.

Using the 460T

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