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5-21
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 IPv4
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 IPv4 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 concurrently 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 5-33.)
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 5-4 on page 5-17 illustrates this scenario.)
Note On the switches covered in this guide, DSCP policies (codepoint re-marking)
cannot be applied to outbound IPv4 packets having IP options. (The 802.1p
priority in the VLAN tag is applied.) For more information on packet criteria
and restrictions, refer to table 5-11 on page 5-41.
Syntax: qos type-of-service diff-services
Enables ToS diff-services.
Syntax: 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 5-33.)
Syntax: no qos type-of-service