Tagged Frames; Priority Packet - Intel 470F - NetStructure Switch User Manual

C H A P T E
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470T Ch2.p65
R 2 Intel® NetStructure

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. The tag can carry priority information,
VLAN information, or both and allows bridges to intelligently direct traffic
across the network.
Some devices don't recognize the tagged Ethernet frames. These devices see
a frame that is too big, and then discard it. When operating 802.1Q (tag-
based) VLANs, there is a way to configure the switch to work with these
untagged devices. For more information see "How to configure 802.1Q
VLANs" in Chapter 5.

Priority Packet

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 or switch and works
with other elements of the network (switches, routers) to deliver packets
based on priority. The priority level can range from 0 (low) to 7 (high).
The 470 switches can read the priority tags and forward traffic on a per port
basis. The switches have two priority queues per port and queue the packet
based on its priority level. For example, when a packet comes into a switch
with a high priority tag, the switch inserts the packet in its high-priority
queue.
Even though there are eight priority levels, the 470 switches can only put a
packet into one of the two queues. The switch maps levels 0-3 to the low
queue and levels 4-7 to the high queue. If a packet is untagged, the switch
can be set to use either the high or low queue for that port. The
470 switches preserve the priority level of the packet.
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