Download Cisco 2970 - Catalyst Switch Specifications

Chapter 27
Configuring QoS
Configuring Standard QoS
Standard QoS Configuration Guidelines
Before beginning the QoS configuration, you should be aware of this information:
•
You configure QoS only on physical ports; there is no support for it on the VLAN or switch virtual
interface level.
•
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 per class map and only one match class-map configuration command per class map
are supported. The ACL can have multiple ACEs, which match fields against the contents of the
packet.
•
Incoming traffic is classified, policed, and marked down (if configured) regardless of whether the traffic
is bridged, routed, or sent to the CPU. It is possible for bridged frames to be dropped or to have their
DSCP and CoS values modified.
•
Only one policer is applied to a packet on an ingress interface. Only the average rate and committed
burst parameters are configurable.
•
The port ASIC device, which controls more than one physical port, supports 256 policers (255
policers plus 1 no policer). The maximum number of policers supported per port is 64. 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.
•
On an interface configured for QoS, all traffic received through the interface is classified, policed,
and marked according to the policy map attached to the interface. On a trunk interface configured
for QoS, traffic in all VLANs received through the interface is classified, policed, and marked
according to the policy map attached to the interface.
•
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.
•
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.
•
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.
Catalyst 2970 Switch Software Configuration Guide
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