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 6-5.)

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 are assigned to the outbound queue

 

priorities as shown in table 6-1above, they still can carry an 802.1p

 

priority for use by downstream devices that implement different

 

queue-priority structures. 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

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 following maximum number of priority

 

rules and/or DSCP policies, in any combination, are allowed:

 

•2510G Switches: 120

 

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

 

 

6-7