4000 Series Switches

4000 Series Switches
If your 4000 switch has been upgraded to run IOS, then the classification, marking, and forwarding
of packets is the same as for the 3550. But when running the legacy CatOS operating
system, the QoS options available for the 4000 series switches are relatively unsophisticated.
This section describes the CatOS QoS options.
Each transmit port has three possible queues. There is one non-configurable queue, and there
are two queues where some configuration is possible. The drop thresholds can be configured,
but tail-drop occurs in all cases when the queue is full.
The switch has a default 802.1p CoS of 0 (zero), but this can be changed. In that case, all
unmarked frames entering the switch are marked with the specified CoS value. Marked frames
cannot be changed.
The default condition is for QoS to be disabled, so first you have to enable QoS on the switch.
Take care that any configuration changes are carried out at an appropriate time, because some
of them will reset ports, and possibly cause spanning tree instability if the network converges.
You can turn on QoS using the set qos enable command.
The port type is defined by the number of transmit queues and the number of drop thresholds
that are supported on the port. For example, the 2q1t port type supports two transmit queues
each with a single configurable drop threshold.
Port types on the Catalyst 4000 are dependent upon the hardware. Use the show
port capabilities command to find out what port type you are configuring.
To configure the CoS mapping and set the thresholds on a configurable port, use the set
qos map port_type q# threshold# cos cos_list. The port type you will already know.
You need to decide which threshold to apply to which queue, and the CoS values to map to the

specified transmit queue. The following example shows the two queues on a 2q1t port being
configured, one with CoS 2-4 and the other with CoS 5-7:
Terry_4000> (enable) set qos map 2q1t 1 1 cos 2-4
Terry_4000> (enable) set qos map 2q1t 2 1 cos 5-7
Qos tx priority queue and threshold mapped to cos successfully.
Terry_4000> (enable)
To view the QoS configuration, use the show qos info config command.