Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual page 633

Chapter 38 Configuring PFC QoS
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You want voice traffic mapped to the strict priority queue, which is queue 4 on 1p3q8t ports. The
example maps the DSCP 46 voice traffic to CoS 5, which means that you want the CoS 5 traffic to be
mapped to the strict priority queue, and you can use the output of the show queueing interface command
to verify that CoS 5 traffic is mapped to the strict priority queue.
This is a list of the queue mappings for all of the traffic types in this example:
Traffic Type
Voice
Voice signaling
PC SAP
Other traffic
Traffic that is transmitted through the switch is directed to these different queues (or "traffic lanes")
based on priority. Because there are more CoS values (zero through seven) than egress queues (three per
interface in this example), there are drop thresholds in each standard (that is, nonstrict priority) queue.
When more than one CoS value is assigned to a given queue, different drop thresholds can be assigned
to these CoS values to distinguish between the different priorities. The thresholds specify the maximum
percentage of the queue that traffic with a given CoS value can use before additional traffic with that
CoS value is dropped. The example only uses three QoS values (high, medium, and low), so you can
assign each CoS value to a separate queue and use the default 100-percent drop thresholds.
You can change the DCSP-to-CoS and CoS-to-queue mapping to suit the needs of your particular
network. Only minor changes are typically necessary, and this example includes no changes. If your
network requires different mapping, see the
Thresholds" section on page
Now you understand how traffic is assigned to the available queues on the output ports of the switch.
The next concept to understand is how the queue weights operate, which is called the queue scheduling
algorithm.
On the Catalyst 6500 series switch, the scheduling algorithms used on the LAN switching modules are
strict priority (SP) queueing and weighted round robin (WRR) queueing. These algorithms determine
the order, or the priority, that the various queues on a port are serviced.
The strict priority queueing algorithm is simple. One queue has absolute priority over all of the other
queues. Whenever there is a packet in the SP queue, the scheduler will service that queue, which ensures
the highest possibility of transmitting the packet and the lowest possible latency in transmission even in
periods of congestion. The strict priority queue is ideal for voice traffic because voice traffic requires the
highest priority and lowest latency on a network, and it also is a relatively low-bandwidth traffic type,
which means that voice traffic is not likely to consume all available bandwidth on a port. You would not
want to assign a high-bandwidth application (for example, FTP) to the strict priority queue because the
FTP traffic could consume all of the bandwidth available to the port, starving the other traffic classes.
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1 5
DSCP CoS (from DSCP-to-CoS map)
46 5
24 3
25 3
0 0
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Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
"Mapping CoS Values to Standard Transmit-Queue
Common QoS Scenarios
Output Queue
Strict Priority
Queue 2, Threshold 2
Queue 2, Threshold 2
Queue 1, Threshold 1
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