Cisco Systems OL-4266-08 manual Traffic Type, CoS from DSCP-to-CoS map Output Queue, 42-119

Chapter 42 Configuring PFC QoS

Common QoS Scenarios

3	5
3	6
3	7

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41 5

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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	DSCP	CoS (from DSCP-to-CoS map)	Output Queue
Voice	46	5	Strict Priority
Voice signaling	24	3	Queue 2, Threshold 2
PC SAP	25	3	Queue 2, Threshold 2
Other traffic	0	0	Queue 1, Threshold 1

Traffic that is transmitted through the router 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 “Mapping CoS Values to Standard Transmit-Queue Thresholds” section on page 42-105.

Now you understand how traffic is assigned to the available queues on the output ports of the router. The next concept to understand is how the queue weights operate, which is called the queue scheduling algorithm.

On the Cisco 7600 series router, 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.

	Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08		42-119