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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter35 Configuring QoS
Configuring Standard QoS
Policing Guidelines
•The port ASIC device, which controls more than one physical port, supports 256 policers
(255 user-configurable policers plus 1 policer reserved for system internal use). The maximum
number of user-configurable policers supported per port is 63. For example, you could configure 32
policers on a Gigabit Ethernet port and 8 policers on a Fast Ethernet port, or you could configure 64
policers on a Gigabit Ethernet port and 5 policers on a Fast Ethernet port. Policers are allocated on
demand by the software and are constrained by the hardware and ASIC boundaries. You cannot
reserve policers per port; there is no guarantee that a port will be assigned to any policer.
•Only one policer is applied to a packet on an ingress port. Only the average rate and committed burst
parameters are configurable.
•You cannot configure policing on the 10-Gigabit interfaces.
•You can create an aggregate policer that is shared by multiple traffic classes within the same
nonhierarchical policy map. However, you cannot use the aggregate policer across different policy
maps.
•On a port configured for QoS, all traffic received through the port is classified, policed, and marked
according to the policy map attached to the port. On a trunk port configured for QoS, traffic in all
VLANs received through the port is classified, policed, and marked according to the policy map
attached to the port.
•If you have EtherChannel ports configured on your switch, you must configure QoS classification,
policing, mapping, and queueing on the individual physical ports that comprise the EtherChannel.
You must decide whether the QoS configuration should match on all ports in the EtherChannel.
•If you need to modify a policy map of an existing QoS policy, first remove the policy map from all
interfaces, and then modify or copy the policy map. After you finish the modification, apply the
modified policy map to the interfaces. If you do not first remove the policy map from all interfaces,
high CPU usage can occur, which, in turn, can cause the console to pause for a very long time.
General QoS Guidelines
These are general QoS guidelines:
•Control traffic (such as spanning-tree bridge protocol data units [BPDUs] and routing update
packets) received by the switch are subject to all ingress QoS processing.
•You are likely to lose data when you change queue settings; therefore, try to make changes when
traffic is at a minimum.
A switch that is running the IP services image supports QoS DSCP and IP precedence matching in
policy-based routing (PBR) route maps with these limitations:
•You cannot apply QoS DSCP mutation maps and PBR route maps to the same interface.
•You cannot configure DSCP transparency and PBR DSCP route maps on the same switch.