Tagged Frames; Priority - Intel Express 530T User Manual

C H A P T E R
®
2 Using the Intel Express 530T Switch

Tagged Frames

The 802.1D (1998 Edition) and 802.1Q specifications published by the
IEEE (Institute of Electrical and Electronic Engineers) extends 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.
For outgoing (egress) packets you can specify whether you want the
packets to be tagged or untagged. For incoming (ingress) packets you
can set the ingress filtering so that packets are forwarded to a specific
port as long as that port is a member of the VLAN. You can set a Port
VLAN Identifier (PVID) so that if untagged traffic goes to that port, the
packet inherits the VLAN Identifier (VID) of the port.

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 530T 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 530T switch can route a
packet into only one of the two queues. The switch maps levels 0-3 to the
low queue (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 530T
Incoming
packet
7
HIGH
6
5
4
Transmit
queue
for the
port
3
2
1
LOW
0
Network
15