Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

A QoS configuration enables you to set the outbound priority queue to which a packet is sent. (In an 802.1Q VLAN environment with VLAN- tagged ports, if QoS is not configured on the switch, but is configured on an upstream device, the priorities carried in the packets determine the forwarding queues in the switch.)

■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 5-6.)

• 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 3, and can be used by

 

downstream devices having up to four outbound port queues. Thus,

 

while packets within the switch move at the four priority levels shown

 

in table 5-1,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 in 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.

 

 

N o t e s

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.

 

 

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