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Quality of Service: Managing Bandwidth More Effectively
Advanced Classifier-Based QoS
Classifier-Based QoS Restrictions
The following restrictions apply to QoS policies configured with the classifier-
based model:
■ You cannot apply a classifier-based QoS policy on a port or VLAN interface
on which a classifier-based QoS policy is already configured. You can,
however, apply a classifier-based policy of a different type, such as port
mirroring.
■ A QoS policy that uses the rate-limit command is not supported on a port
interface on which ICMP rate-limiting has already been globally config-
ured. To apply the QoS policy, you must first disable the ICMP rate-limiting
configuration. For more information, refer to the “Configuring ICMP”
section in the “Configuring IP Parameters for Routing Switches” chapter
in the Multicast and Routing Guide.
In cases where you want to maintain an ICMP rate-limiting configuration,
configure a QoS policy in which you add the necessary match statements
for the ICMP traffic in a class configuration, and then configure a rate-limit
action for the class in the policy configuration.
■ In a QoS policy that uses the class action rate-limit command, the rate-limit
is calculated on a per-module or per port-bank basis. If trunked ports or
VLANs with a configured rate-limit span multiple modules or port-banks,
the configured rate-limit is not guaranteed.
■ In a QoS policy that uses the class action dscp command, the DSCP value
you enter must be already configured with an 802.1p priority in the DSCP
Policy table (see Table 6-11).
Interaction with Other Software Features
After you apply a QoS policy to an interface, you may receive an error message
if there are not sufficient hardware resources to support the policy. In this
case, use the show resources command to verify the amount of resources that
are currently in use and the resources available on the switch. QoS policies
share the same hardware resources with other software features, such as
mirroring policies, ACLs, virus throttling, the management VLAN, and so on.
Use the displayed information to decide if you need to re-prioritize current
resource usage by reconfiguring or disabling software features to free the
resources reserved for less important features.
For more information, refer to “Displaying Resource Usage for QoS Policies”
on page 6-32 and the “Monitoring Resources” chapter in the Management and
Configuration Guide.
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