Cisco Systems OL-4266-08 manual 42-118

Chapter 42 Configuring PFC QoS

Common QoS Scenarios

To assign traffic to these queues, you need to configure a mapping of priority values to queues. QoS uses the DSCP-to-CoS map to map the 64 possible outgoing DSCP values to the eight possible 802.1p values, and then uses a CoS-to-queue map to map the CoS values to queues.

When the packet enters the router, QoS is either configured to classify and mark the packet with a configured DSCP value (as in the “Classifying Traffic from PCs and IP Phones in the Access Layer” section on page 42-113) or to trust the packet’s incoming DSCP value (as in the “Accepting the Traffic Priority Value on Interswitch Links” section on page 42-116). These options determine the packet’s priority as it leaves the router.

This example shows how to display the DSCP-to-CoS mapping:

Router#	show	mls qos maps				dscp-cos
Dscp-cos map:										(dscp= d1d2)
d1 : d2 0 1 2 3						4	5	6 7 8 9
-------------------------------------
0	:	00	00	00	00	00	00	00 00	01	01
1	:	01	01	01	01	01	01	02 02	02	02
2	:	02	02	02	02	03	03	03 03	03	03
3	:	03	03	04	04	04	04	04 04	04	04
4	:	05	05	05	05	05	05	05 05	06	06
5	:	06	06	06	06	06	06	07 07	07	07
6	:	07	07	07	07
Router#

The example marked the voice traffic with a DSCP value of 46. You can use the command output to translate DSCP 46 to CoS 5. You can use the command output to translate the other marked DSCP values to CoS values.

You can make changes to this mapping table to suit the needs of your particular network. Only minor changes are typically necessary; this example does not make any changes.

For queueing purposes, the configuration derives a CoS value from the outgoing DSCP value. This CoS value is used for queue assignment even if the outgoing port is an access port (that is, not a trunk port). However, there will be no 802.1q VLAN tag transmitted on the network if the outgoing port is an access port.

Map each derived CoS value to the queue structure. This example shows how to display the default CoS-to-queue mapping, which shows the queue to which each of the eight CoS values is mapped:

Router# show queueing interface gigabitethernet 5/1 begin cos-map

queue thresh cos-map

---------------------------------------

1			1	0
1			2	1
1			3
1			4
1			5
1			6
1			7
1			8
2			1	2
2			2	3	4
2			3
2			4
2			5
2			6
2			7
2			8
3			1	6	7
3			2
3			3
3			4
		Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
	42-118				OL-4266-08