Quality of Service (QoS): Managing Bandwidth More Effectively

Using QoS Classifiers To Configure QoS for Outbound Traffic

The specified DSCP policies overwrite the original DSCPs on the selected packets, and use the 802.1p priorities previously configured in the DSCP policies in step 2.

Figure 6-20. Example of Policy Assignment to Outbound Packets on the Basis of the DSCP in the Packets Received from Upstream Devices

Details of QoS IP Type-of-Service

IP packets include a Type of Service (ToS) byte. The ToS byte includes:

A Differentiated Services Codepoint (DSCP): This element is com­ prised of the upper six bits of the ToS byte). There are 64 possible codepoints. In the switches covered by this manual, the default qos configuration includes the codepoint having the 802.1p priority setting for Expedited Forwarding, while all others, including the Assured-Forward­ ing codepoints, are unused (and listed with No-overridefor a Priority).

Refer to figure 6-11on page 6-59for an illustration of the default DSCP policy table.

Using the qos dscp map command, you can configure the switch to assign different prioritization policies to IPv4 packets having different code- points. As an alternative, you can configure the switch to assign a new codepoint to an IPv4 packet, along with a corresponding 802.1p priority (0-7). To use this option in the simplest case, you would:

a. Configure a specific DSCP with a specific priority in an edge switch.

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