Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

You can configure a QoS priority of 0 through 3 for an outbound packet. When the packet is then sent to a port, the QoS priority determines which outbound queue the packet uses:

Table 5-2. QoS Priority Settings and Operation

QoS Priority Setting

Outbound Port Queue

 

 

1-2

low priority (1,2)

 

 

0-3

normal priority (3,4)

 

 

4-5

medium priority (5,6)

 

 

6-7

high priority (7,8)

 

 

If a packet is not in a VLAN-tagged port environment, then the QoS settings in table 5-2 control only to which outbound queue the packet goes. Without VLAN tagging, no 802.1p priority is added to the packet for downstream device use. But if the packet is in a VLAN-tagged environment, then the above setting is also added to the packet as an 802.1p priority for use by downstream devices and applications (shown in table 5-3). In either case, an IP packet can also carry a priority policy to downstream devices by using DSCP-marking in the ToS byte.

Table 5-3. Mapping Switch QoS Priority Settings to Device Queues

 

 

Priority Setting

Outbound Port

802.1p Priority Setting Added to

Queue

 

 

 

Queues in the

Tagged VLAN Packets Leaving

Assignment in

 

 

 

Switch

 

the Switch

Downstream

 

 

 

 

 

 

Devices With:

 

 

 

 

 

 

4 Queues

 

 

 

 

 

 

 

 

 

1

Queue 1

1

(low priority)

Queue 1

 

2

2

 

 

 

 

 

 

0

 

 

 

 

 

Queue 2

0

(normal priority)

Queue 2

 

3

3

 

 

 

 

 

 

4

 

 

 

 

 

Queue 3

4

(medium priority)

Queue 3

 

5

5

 

 

 

 

 

 

6

 

 

 

 

 

Queue 4

6

(high priority)

Queue 4

 

7

7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N o t e

 

The number of outbound port queues in the switch is fixed at four.

 

 

 

 

 

 

 

5-9