13-8
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter 13 Configuring QoS
Understanding QoS
Queueing and Scheduling
Note Both the enhanced and standard software images support this feature.
The Catalyst 2950 switches provide QoS-based 802.1P CoS values. QoS uses classification and
scheduling to transmit network traffic from the switch in a predictable manner. QoS classifies frames by
assigning priority-indexed CoS values to them and gives preference to higher-priority traffic such as
telephone calls.
How Class of Service Works
Before you set up 802.1P CoS on a Catalyst 2950 that operates with the Catalyst 6000 family of switches,
refer to the Catalyst 6000 documentation. There are differences in the 802.1P implementation, and they
should be understood to ensure compatibility.
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 transmitted 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 CMS. 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 Catalyst 2950 switches (802.1P user priority) have four priority queues. The frames are forwarded
to appropriate queues based on priority-to-queue mapping that you defined.
CoS and WRR
The Catalyst 2950 switches support 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. Queues can have priorities from 0 to 7,
7 being the highest. Packets in the high-priority queue always transmit first, and packets in the
low-priority queue do not transmit until all the high-priority queues become empty.
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