Prerequisites for QoS
Before configuring standard QoS, you must have a thorough understanding of these items:
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The types of applications used and the traffic patterns on your network.
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Traffic characteristics and needs of your network. For example, is the traffic on your network bursty? Do you need to reserve bandwidth for voice and video streams?
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Bandwidth requirements and speed of the network.
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Location of congestion points in the network.
QoS ACL Guidelines
Follow these guidelines when configuring QoS with access control lists (ACLs):
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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.
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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.
Policing Guidelines
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Only one policer is applied to a packet on an ingress port. Only the average rate and committed burst parameters are configurable.
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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.
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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.
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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 the general QoS guidelines:
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You configure QoS only on physical ports; there is no support for it at the VLAN level and EtherChannel ports.
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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.
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You are likely to lose data when you change queue settings; therefore, try to make changes when traffic is at a minimum.