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Quality of Service (QoS): Managing Bandwidth More Effectively

Using QoS Classifiers To Configure QoS for Outbound Traffic

On switches covered in this guide, “mixing” ToS DSCP policies and 802.1p priorities is not recommended. Refer to the Note on page 6-10.

For more on DSCP, refer to “Terminology” on page 6-6.

Steps for Creating a Policy Based on VLAN-ID Classifier.

1.Determine the VLAN-ID classifier to which you want to assign a DSCP policy.

2.Determine the DSCP policy for packets carrying the selected VLAN-ID:

a.Determine the DSCP you want to assign to the selected packets. (This codepoint will be used to overwrite the DSCP carried in packets received from upstream devices.)

b.Determine the 802.1p priority you want to assign to the DSCP.

3.Configure the DSCP policy by using qos dscp-mapto configure the priority for each codepoint. (For details, see the example later in this section, and to “Differentiated Services Codepoint (DSCP) Mapping” on page 6-58.)

A codepoint must have an 802.1p priority (0 - 7) before you can configure the codepoint for use in prioritizing packets by VLAN-ID. If a codepoint you want to use shows No-overridein the Priority column of the DSCP Policy table (show qos dscp-map), then assign a priority before proceeding.

4.Configure the switch to assign the DSCP policy to packets with the specified VLAN-ID.

Syntax: qos dscp-map < codepoint > priority < 0 - 7 >

This command is optional if a priority has already been assigned to the < codepoint >. The command creates a DSCP policy by assigning an 802.1p priority to a specific DSCP. When the switch applies this priority to a packet, the priority determines the packet’s queue in the outbound port to which it is sent. If the packet leaves the switch on a tagged port, it carries the 802.1p priority with it to the next downstream device. If the packet is IPv4, the packet’s DSCP will be replaced by the codepoint specified in this command. (Default: For most codepoints, No-override. See figure 6-11 on page 6-59on page 6-59.)

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