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QoS is an integral part of any multilayer switched network deployment. With QoS, you can build a network of predictable behavior for latency, jitter, and packet loss. In addition, QoS mitigates anomalous network behavior and provides for differentiation of traffic flows. The following list summarizes QoS:

  • Classification associates a priority value to a frame.

  • Catalyst switches support classification on a per-interface or per-packet basis using ACLs.

  • Marking changes the DSCP value of a packet or CoS value of a frame on ingress or egress.

  • Catalyst switches use policing or shaping to condition traffic rates.

  • Catalyst switches support congestion management through the use of WRR, sharing, and shaping scheduling mechanisms.

  • Congestion avoidance uses WRED on Catalyst switches to improve link utilization under congestion with TCP flows.

In brief, adhere to the following guidelines and recommendations for deploying QoS in a multilayer switched network:

  • Before configuring Catalyst switches for QoS, develop a QoS plan that clearly outlines the necessary traffic classification.

  • Opt to classify traffic based on DSCP values instead of CoS values.

  • Apply classification as close to the edge as possible, preferably in the Building Access submodule.

  • Trust interfaces that interconnect switches where classification configuration already exists.

  • Use policing to effectively condition traffic data rates.

  • Apply congestion management to all interfaces and use priority queuing on interfaces that transmit VoIP traffic.

  • Use congestion avoidance on interfaces where heavy congestion exists with TCP flows.

For additional information on QoS, refer to the following resources:

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