Tagged Frames; Priority - Intel ES460MSX - Ethernet 1000Base SX Switch Module User Manual

C H A P T E
460T Ch2.p65
R 2 Using the Express 460T 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 allows 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 Express 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.
Even though there are eight priority levels, the Express 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.
Express 460T
Incoming
packet
15
7
HIGH
6
5
4
transmit
queue
for the
port
3
2
1
LOW
0
4/19/00, 4:09 PM
Network
15