26-40
Catalyst 3750 Metro Switch Software Configuration Guide
78-15870-01
Chapter 26 Configuring QoS
Configuring Standard QoS
Standard QoS Configuration Guidelines
Before beginning the QoS configuration, you should be aware of this information:
• It is not possible to match IP fragments against configured IP extended ACLs to enforce QoS. IP
fragments are sent as best-effort. IP fragments are denoted by fields in the IP header.
• Only one ACL can be configured per class map. The ACL can have multiple ACEs, which match
fields against the contents of the packet. Class maps that contain ACLs are supported only in an
ingress policy-map.
• Only one policy map per port is supported. You can attach one ingress service-policy per port and
one egress service-policy per ES port.
• Inbound traffic is classified, policed, and marked down (if configured) regardless of whether the traffic
is bridged, routed, or sent to the CPU. Bridged frames can be dropped or have their DSCP and CoS values
modified.
• Only one ingress policer is applied to a packet on a port. Only the average-rate and committed-burst
parameters are configurable.
• You can create an aggregate policer that is shared by multiple traffic classes within the same policy
map. However, you cannot use the aggregate policer across different policy maps.
• On a standard port and on the input of an ES port, the port ASIC device, which controls more than
one physical port, supports 256 ingress policers (255 policers plus 1 no policer). The maximum
number of policers supported per is 64. 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. These limitations do not apply
to egress policers configured on the ES ports.
• 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.
• Because the switch does not support attaching a service policy to a logical interface (such as an
EtherChannel), 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.
• 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.
For outbound traffic on an ES port, see the “Hierarchical QoS Configuration Guidelines” section on
page 26-76.
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