Campus QoS Implementation

• A robust, modern switching design is a requirement.

Campus QoS General Guidelines:

• A robust, modern switching design is a requirement.

• Buffer management is more of a concern than bandwidth management.

• Multiple queues are required on all interfaces to prevent Tx queue congestion and drops.

• Voice traffic should always go into the highest priority queue.

• Trust Cisco IP Phone CoS setting but not the PC CoS setting.

• Classify and mark traffic as close to the source as possible.

• Use class-based policing to rate-limit certain unwanted excess traffic.

IP telephony, videoconferencing, e-learning, and mission-critical data applications are becoming more common in enterprise networks. QoS functions such as classification, scheduling, and provisioning are required within the campus to manage the switch output buffers to minimize packet loss, delay, and jitter. Some of the general guidelines when implementing campus QoS include the following:

■ Use a hierarchical design model for the campus network design, consisting of three layers: access layer, distribution layer, and core layer.

■ Use multiple queues on the transmit interfaces to minimize the potential for dropped or delayed traffic caused by transmit buffer congestion.

■ Give real-time voice and video traffic priority over data traffic.

■ Establish proper trust boundaries. For example, at the access layer switches, only trust the IP Phone CoS marking, not the PC CoS marking.

■ Classify and mark the traffic as soon as possible. For example, use class-based markings at the access layer switches to classify and mark the traffic coming into the network. NBAR can be used on the access or distribution switches to classify traffic if the switch supports NBAR.

■ At the access or distribution layers, use class-based policing to limit any undesired non-business-related excess traffic.

■ Do not classify or mark packets at the core layer, because this slows down traffic.

■ Do most of the complex QoS configurations, such as traffic shaping, header compression, link fragmentation and interleaving (LFI), LLQ, WRED, and re-marking of traffic, at the WAN CE router.

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