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5-8
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
A QoS configuration enables you to set the outbound priority queue to
which a packet is sent. (In an 802.1Q VLAN environment with VLAN-
tagged ports, if QoS is not configured on the switch, but is configured on
an upstream device, the priorities carried in the packets determine the
forwarding queues in the switch.)
■ Configuring a priority for outbound packets and a service (prior-
ity) policy for use by downstream devices:
• DSCP Policy: This feature enables you to set a priority policy in
outbound IP packets. (You can configure downstream devices to read
and use this policy.) This method is not dependent on VLAN-tagged
ports to carry priority policy to downstream devices, and can:
– Change the codepoint (the upper six bits) in the ToS byte.
– Set a new 802.1p priority for the packet.
(Setting DSCP policies requires IPv4 inbound packets. Refer to the
“IPv4” entry under “Terminology” on page 5-6.)
• 802.1p Priority Rules: An outbound, VLAN-tagged packet carries
an 802.1p priority setting that was configured (or preserved) in the
switch. This priority setting ranges from 0 to 3, and can be used by
downstream devices having up to four outbound port queues. Thus,
while packets within the switch move at the four priority levels shown
in table 5-1, above, they still can carry an 802.1p priority that can be
used by downstream devices having more or less than the four priority
levels in the switches covered in this guide. Also, if the packet enters
the switch with an 802.1p priority setting, QoS can override this
setting if configured with an 802.1p priority rule to do so.
Notes If your network uses only one VLAN (and therefore does not require
VLAN-tagged ports) you can still preserve 802.1p priority settings in
your traffic by configuring the ports as tagged VLAN members on the
links between devices you want to honor traffic priorities.