Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

■Configuring a priority for outbound packets and a service (prior­ ity) policy for use by downstream devices:

•DSCP Policy: This feature enables you to set a priority policy in outbound IP packets. (You can configure downstream devices to read and use this policy.) This method is not dependent on VLAN-tagged ports to carry priority policy to downstream devices, and can:

–Change the codepoint (the upper six bits) in the ToS byte.

–Set a new 802.1p priority for the packet.

(Setting DSCP policies requires IPv4 inbound packets. Refer to the “IPv4” entry under “Terminology” on page 6-6.)

•802.1p Priority Rules: An outbound, VLAN-tagged packet carries an 802.1p priority setting that was configured (or preserved) in the switch. This priority setting ranges from 0 to 7, and can be used by downstream devices having up to eight outbound port queues. Thus, while packets within the switch move at the four priority levels shown in table 6-1,above, they still can carry an 802.1p priority that can be used by downstream devices having more or less than the four priority levels in the switches covered by this guide. Also, if the packet enters the switch with an 802.1p priority setting, QoS can override this setting if configured with an 802.1p priority rule to do so.

If your network uses only one VLAN (and therefore does not require VLAN-tagged ports) you can still preserve 802.1p priority settings in your traffic by configuring the ports as tagged VLAN members on the links between devices you want to honor traffic priorities.

■Rule and Policy Limits: The switches covered by this guide allow a maximum of 128 priority rules and/or DSCP policies per QoS feature.

You can configure a QoS priority of 0 through 7 for an outbound packet. When the packet is then sent to a port, the QoS priority determines which outbound queue the packet uses:

Table 6-2. QoS Priority Settings and Operation

QoS Priority Setting

Outbound Port Queue

 

 

1 - 2

low priority

 

 

0 - 3

normal priority

 

 

4 - 5

medium priority

6 - 7

high priority

 

 

If a packet is not in a VLAN-tagged port environment, then the QoS settings in table 6-2 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.

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