6-7
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
■ 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 6-5.)
• 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 7, and can be used by
downstream devices having up to eight outbound port queues. Thus,
while packets within the switch are assigned to the outbound queue
priorities as shown in table 6-1 above, they still can carry an 802.1p
priority for use by downstream devices that implement different
queue-priority structures. 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.
Note 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.
Rule and Policy Limits: The following maximum number of priority
rules and/or DSCP policies, in any combination, are allowed:
•2510G Switches: 120
You can configure a QoS priority of 0 through 7 for an outbound packet. When
the packet is then sent to a port, the QoS priority determines which outbound
queue the packet uses:
Table 6-2. QoS Priority Settings and Operation
QoS Priority Setting Outbound Port Queue
1 - 2 low priority
0 - 3 normal priority
4 - 5 medium priority
6 - 7 high priority
