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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 29 Configuring QoS
Understanding QoS
Port Priority
Frames received from users in the administratively defined VLANs are classified or tagged for
transmission to other devices. Based on rules that you define, a unique identifier (the tag) is inserted in
each frame header before it is forwarded. The tag is examined and understood by each device before any
broadcasts or transmissions to other switches, routers, or end stations. When the frame reaches the last
switch or router, the tag is removed before the frame is sent to the target end station. VLANs that are
assigned on trunk or access ports without identification or a tag are called native or untagged frames.
For IEEE 802.1Q frames with tag information, the priority value from the header frame is used. For
native frames, the default priority of the input port is used.
Port Scheduling
Each port on the switch has a single receive queue buffer (the ingress port) for incoming traffic. When
an untagged frame arrives, it is assigned the value of the port as its port default priority. You assign this
value by using the CLI or . A tagged frame continues to use its assigned CoS value when it passes
through the ingress port.
CoS configures each transmit port (the egress port) with a normal-priority transmit queue and a
high-priority transmit queue, depending on the frame tag or the port information. Frames in the
normal-priority queue are forwarded only after frames in the high-priority queue are forwarded.
The switch (IEEE 802.1P user priority) has four priority queues. The frames are forwarded to
appropriate queues based on the priority-to-queue mapping that you defined.
Egress CoS Queues
The switch supports four CoS queues for each egress port. For each queue, you can specify these types
of scheduling:
• Strict priority scheduling
Strict priority scheduling is based on the priority of queues. Packets in the high-priority queue are
always sent first, and packets in the low-priority queue are not sent until all the high-priority queues
become empty.
The default scheduling method is strict priority.
• Weighted round-robin (WRR) scheduling
WRR scheduling requires you to specify a number that indicates the importance (weight) of the
queue relative to the other CoS queues. WRR scheduling prevents the low-priority queues from
being completely neglected during periods of high-priority traffic. The WRR scheduler sends some
packets from each queue in turn. The number of packets it sends corresponds to the relative
importance of the queue. For example, if one queue has a weight of 3 and another has a weight of 4,
three packets are sent from the first queue for every four that are sent from the second queue. By
using this scheduling, low-priority queues have the opportunity to send packets even though the
high-priority queues are not empty.
• Strict priority and WRR scheduling
Strict priority and WRR scheduling, also referred to as strict priority queueing, uses one of the
egress queues as an expedite queue (queue 4). The remaining queues participate in WRR. When the
expedite queue is configured, it is a priority queue and is serviced until it is empty before the other
queues are serviced by WRR scheduling.