228 Cisco LAN Switching Configuration Handbook
Tip The QoS operations on a Catalyst 2900XL or 3500XL switch are limited to QoS
trust and fixed-queue scheduling. Therefore, these switches are presented here separately.
1. (Optional) Classify traffic based on a port.
a. (Optional) Set the default ingress CoS value:
(interface) switchport priority default cos
Frames that are untagged receive CoS value cos (0 to 7).
b. (Optional) Don’t trust any inbound information:
(interface) switchport priority override
For a Catalyst 200/3000, the CoS value is set to the default CoS value configured
in Step 1a. By default, all switch ports override inbound untagged or static access
CoS values with 0.
c. (Optional) Instruct a connected appliance to handle CoS:
(interface) switchport priority extend {cos cos | none | trust}
CoS trust can be extended to a Cisco IP Phone or other appliance that is con-
nected to a Catalyst switch port. The switch can instruct the appliance on how to
trust CoS values from other devices connected to it. CoS trust can be cos (over-
ride the CoS in frames from other devices with value cos, 0 to 7), none (the
appliance doesn’t do anything with the CoS, the default), or trust (the appliance
trusts and forwards the CoS in frames from other devices). See Chapter 14,
“Voice,” for more IP Phone configuration information.
2. Port queue scheduling:
■ Catalyst 2000/3000 switches have a single ingress queue. This queue cannot be
configured.
■ These switches have 2q0t egress ports. Frames with a CoS 0 to 3 are assigned to
the lower-priority queue (queue 1). Frames with CoS 4 to 7 are assigned to the high-
er-priority queue (queue 2).
■ The egress queue scheduling is not configurable. As well, the congestion-avoid-
ance thresholds are fixed at 100 percent.
All Other Catalyst Configuration
1. Enable QoS functionality:
(global) mls qos
By default, QoS is disabled. All traffic is switched in a “pass-through” mode, where
only “best effort” delivery is offered.
Catalyst 2000/3000 Configuration
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