Default CoS assignment can be altered during configuration

The default QoS settings for the Catalyst 2950 and 3550 switches are as follows:

■ The CoS value of 0 is assigned to all incoming packets.

■ The default port trust state is untrusted. If a port is connected to an IP Phone, you should change the default port configuration to trust the CoS setting from the IP Phone using the mls qos trust command.

■ No policy maps are configured.

■ No policers are configured.

■ The default CoS assignments to priority queues are as follows:

The default CoS assignments to priority queues change once auto qos voip is configured.

■ With auto qos voip [cisco-phone] or [trust], the default CoS assignments change to:

• During QoS processing, the switch represents the priority of all traffic (including non-IP traffic) with an internal DSCP value.

• During classification, QoS uses configurable mapping tables to derive the internal DSCP (a six-bit value) from received CoS value.

Default CoS-to-DSCP Ma

P

CoS value

0

1

2

3

4

5

6

7

DSCP value

0

8

16

24

32

40

48

56 I

• Before the traffic reaches the scheduling stage, QoS uses the configurable DSCP-to-CoS map to derive a CoS value from the internal DSCP value.

Default DSCP-to-CoS Map

DSCP values

0

8,10

16,18

24,26

32,34

40,46

48

56

CoS values

0

1

2

3

4

5

6

Actions at the egress interface include queuing and scheduling:

■ Queuing: Queuing evaluates the internal DSCP and determines which of the four egress queues should be used for placing the packet. The DSCP value is mapped to a CoS value, which selects one of the queues.

■ Scheduling: Scheduling services the four egress queues based on their configured WRR weights and thresholds. One of the queues can be the expedite queue, which is serviced until empty before the other queues are serviced. Congestion avoidance techniques include tail drop and WRED on Gigabit-capable Ethernet ports and tail drop (with only one threshold) on 10/100 Ethernet ports.

During QoS processing, the switch represents the relative-priority of all traffic (including non-IP traffic) with an internal DSCP value. During classification, QoS uses configurable mapping tables to derive the internal DSCP (a six-bit value) from received CoS or IP precedence (three-bit) values. These maps include the CoS-to-DSCP map and the IP-precedence-to-DSCP map.

On an ingress interface configured in the DSCP-trusted state, if the DSCP values are different between the QoS domains, the configurable DSCP-to-DSCP-mutation map can be applied to the interface that is on the boundary between the two QoS domains.

During policing, QoS can assign another DSCP value to an IP or non-IP packet, if the packet is out of profile and the policer specifies a marked-down DSCP value. This configurable map is called the policed-DSCP map.

Before the traffic reaches the scheduling stage, QoS uses the configurable DSCP-to-CoS map to derive a CoS value from the internal DSCP value. Through the CoS-to-egress-queue map, the CoS values select one of the four egress queues for output processing.

The CoS-to-DSCP, DSCP-to-CoS, and the IP-precedence-to-DSCP (Catalyst 3550 only) maps have default values that might or might not be appropriate for the network.

The default DSCP-to-DSCP-mutation (Catalyst 3550 only) map and the default policed-DSCP map are null maps; they map an incoming DSCP value to the same DSCP value.

The DSCP-to-DSCP-mutation map is the only map that can be applied to a specific Gigabit-capable Ethernet port or to a group of 10/100 Ethernet ports. All other maps apply to the entire switch.

The figure provides an example of a CoS value mapped to the DSCP value in a Catalyst 2950 switch.

The trust boundary has been established on the switch port to trust the CoS setting from the IP Phone. By default, the CoS and DSCP value of a packet coming from a Cisco IP Phone is set to CoS 5 and DSCP Expedited Forwarding (EF) value 46.

On the output of the switch, in the Layer 3 header, the DSCP will be set to 40 using the default CoS-to-DSCP map.

©2006 Cisco Systems, Inc. All rights reserved.

The figure shows the previous packet as it arrives to its destination after traversing the network.

In this example, the switch port connecting to the router is set to trust DSCP. Therefore, the Layer 3 header will have a DSCP value of 40 (from the previous slide) and as it traverses the switch, its CoS value is set to 5 using the default DSCP-to-CoS map.

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