5-13
Quality of Service: Managing Bandwidth More Effectively
QoS Traffic Marking
Note You can reconfigure the QoS queue setting to change the number of outbound
port queues in the switch from eight (default) to four or two queues. For more
information, see “QoS Queue Configuration” on page 5-97.
Layer 3 DSCP Marking
By changing or honoring the settings of the DSCP codepoint in IP packet
headers, QoS allows you to control the DSCP and associated 802.1p priority
values in outbound IP packets that are sent to downstream devices.
You can later configure downstream devices to read and use the DSCP policy
that QoS sets. When marking the DSCP bits in IP packets, a QoS policy is not
dependent on VLAN-tagged ports to carry 802.1p packet priorities to down-
stream devices (as shown in Table 5-4).
When you configure a Layer 3 DSCP policy, you specify:
■ Bit values for the DSCP codepoint (the upper six bits in the ToS/Traffic
Class byte in IP packet headers), entered in either binary format, the
decimal equivalent, or an ASCII standard (hexadecimal) name
■ An 802.1p priority value that is associated with the new DSCP bit values
Certain DSCP codepoints (such as Assured Forwarding and Expedited
Forwarding) have default 802.1p priorities as shown in Table 5-11.
A DSCP policy assigns a DSCP codepoint and 802.1p priority value to IPv4 and
IPv6 packets. As shown in Figure 5-2, you can classify traffic on an edge switch
and use Layer 3 DSCP-marking (instead of only 802.1p priority) to assign and
preserve QoS policies on downstream devices. In this case, if you reconfigure
the 802.1p priority associated with the DSCP codepoint, the new 802.1p
assignment takes effect starting on the switch on which it is configured and
is used in packets sent to downstream devices.
3 Queue 4 3 Queue 4
4 Queue 5 4 (medium priority) Queue 5 Queue 3
5 Queue 6 5 Queue 6 Queue 2
6 Queue 7 6 (high priority) Queue 7 Queue 4
7 Queue 8 7 Queue 8
Configured
802.1p Priority
Outbound Port
Queue in the
Switch
802.1p Priority Added to
Tagged VLAN Packets
Exiting the Switch
Queue Assignment in Downstream Devices
With:
8 Queues 4 Queues 2 Queues
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