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Quality of Service (QoS): Managing Bandwidth More Effectively
Using QoS Classifiers To Configure Quality of Service for Outbound Traffic
Assigning a DSCP Policy on the Basis of the DSCP in IP
Packets Received from Upstream Devices
The preceding section describes how to forward a policy set by an edge (or
upstream) switch. This option changes a DSCP policy in an outbound IP
packet by changing its IP ToS codepoint and applying the priority associated
with the new codepoint. (A DSCP policy consists of a differentiated services
codepoint and an associated 802.1p priority.) You can use this option concur-
rently with the diffserv 802.1p priority option (above), as long as the DSCPs
specified in the two options do not match.
To use this option to configure a change in policy:
1. Identify a DSCP used to set a policy in packets received from an upstream
or edge switch.
2. Create a new policy by using qos dscp-map < codepoint > priority < 0 - 7 > to
configure an 802.1p priority for the codepoint you will use to overwrite
the DSCP the packet carries from upstream. (For more on this topic, refer
to
“Differentiated Services Codepoint (DSCP) Mapping” on page 15-51.)
3. Use qos type-of-service diff-services < incoming-DSCP > dscp < outgoing-
DSCP > to change the policy on packets coming from the edge or upstream
switch with the specified incoming DSCP.
(Figure 15-12 on page 15-27 illustrates this scenario.)
Syntax: qos type-of-service diff-services
Enables ToS diff-services.
qos type-of-service diff-services < current-codepoint > dscp
< new-codepoint >
Configures the switch to select an incoming IP packet carry-
ing the <current-codepoint > and then use the <new-codepoint>
to assign a new, previously configured DSCP policy to the
packet. The policy overwrites the <current-codepoint> with the
< new-codepoint > and assigns the 802.1p priority specified
by the policy. (Use the
qos dscp-map command to define the
priority for the DSCPs—page 15-51.)
no qos type-of-service
Disables all ToS classifier operation. Current ToS DSCP
policies and priorities remain in the configuration and will
become available if you re-enable ToS diff-services.
15-31