Cisco Systems 2960 WTD Thresholds, Buffer and Bandwidth Allocation

29-14
Catalyst 2960 Switch SoftwareConfiguration Guide
78-16881-01
Chapter29 Co nfiguring QoS
Understanding QoS
The switch supports two configurable ingress queues, which are serviced by SRR in shared mode only.
Table29-1 describes the queues.
You assign each packet that flows through the switch to a queue and to a threshold. Specifically, you map
DSCP or CoS values to an ingress queue and map DSCP or CoS values to a threshold ID. You use the
mls qos srr-queue input dscp-map queue queue-id {dscp1...dscp8 | threshold threshold-id
dscp1...dscp8} or the mls qos srr-queue input cos-map queue queue-id {cos1...cos8 | threshold
threshold-id cos1...cos8} global configuration command. You can display the DSC P input queue
threshold map and the CoS input queue threshold map by using the show mls qos maps privileged EXEC
command.
WTD Thresholds
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has
three drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit)
threshold preset to the queue-full state. You assign the two explicit WTD threshold percentages for
threshold ID 1 and ID 2 to the ingress queues by using the mls qos srr-queue input threshold queue-id
threshold-percentage1 threshold-percentage2 global configuration command. Each threshold value is a
percentage of the total number of allocated buffers for the queue. The drop threshold for threshold ID 3
is preset to the queue-full state, and you cannot modify it. For more information about how WTD works,
see the “Weighted Tail Drop” section on page29-12.
Buffer and Bandwidth Allocation
You define the ratio (allocate the amount of space) with which to divide the ingress buffers between the
two queues by using the mls qos srr-queue input buffers percentage1 percentage2 global configuration
command. The buffer allocation together with the bandwidth allocation contro l how much data can be
buffered and sent before packets are dropped. You allocate bandwidth as a percentage by using the mls
qos srr-queue input bandwidth weight1 weight2 global configuration command. The ratio of the
weights is the ratio of the frequency in which the SRR scheduler sends packets from each queue.
Table29-1 Ingress Queue Types
Queue Type1
1. The switch uses two nonconfigurable queues for traffic that is essential for proper network operation.
Function
Normal User traffic that is considered to be normal priority. You can configure three different
thresholds to differentiate among the flows. You can use the mls qos srr-queue input
threshold, the mls qos srr-queue input dscp-map, and the mls qos srr-queue input
cos-map global configuration commands.
Expedite High-priority user traffic such as differentiated services (DF) expedited for warding or
voice traffic. You can configure the bandwidth required for this traffic as a percentage
of the total traffic by using the mls qos srr-queue input priority-queue global
configuration command. The expedite queue has guaranteed bandwidth.