Quality of Service (QoS): Managing Bandwidth More Effectively

Using QoS Classifiers To Configure QoS for Outbound Traffic

QoS IP Type-of-Service (ToS) Policy and Priority

 

QoS Classifier Precedence: 3

 

This feature applies only to IPv4 traffic and performs either of the following:

 

■ ToS IP-Precedence Mode: All IP packets generated by upstream devices

 

and applications include precedence bits in the ToS byte. Using this mode,

 

the switch uses these bits to compute and assign the corresponding 802.1p

 

priority.

 

■ ToS Differentiated Services (Diffserv) Mode: This mode requires

 

knowledge of the codepoints set in IP packets by the upstream devices

 

and applications. It uses the ToS codepoint in IP packets coming from

 

upstream devices and applications to assign 802.1p priorities to the pack­

 

ets. You can use this option to do both of the following:

 

• Assign a New Prioritization Policy: A “policy” includes both a

 

codepoint and a corresponding 802.1p priority. This option selects an

 

incoming IPv4 packet on the basis of its codepoint and assigns a new

 

codepoint and corresponding 802.1p priority. (Use the qos dscp-map

 

command to specify a priority for any codepoint—page 6-58.)

 

• Assign an 802.1p Priority: This option reads the DSCP of an

 

incoming IPv4 packet and, without changing this codepoint, assigns

 

the 802.1p priority to the packet, as configured in the DSCP Policy

 

Table (page 6-58). This means that a priority value of 0 - 7 must be

 

configured for a DSCP before the switch will attempt to perform a

 

QoS match on the packet’s DSCP bits.

 

Before configuring the ToS Diffserv mode, you must use the dscp-map

 

command to configure the desired 802.1p priorities for the codepoints you

 

want to use for either option. This command is illustrated in the following

 

examples and is described under “Differentiated Services Codepoint

 

(DSCP) Mapping” on page 6-58.

 

Unless IP-Precedence mode and Diffserv mode are both disabled (the default

 

setting), enabling one automatically disables the other. For more on ToS

 

operation, refer to “Details of QoS IP Type-of-Service” on page 6-43.

 

 

N o t e s

On switches covered in this guide, “mixing” ToS DSCP policies and 802.1p

 

priorities is not recommended. Refer to the Note on page 6-10.

 

IP-ToS QoS does not support layer-2 SAP encapsulation. For more information

 

on packet-type restrictions, refer to ‘‘Details of Packet Criteria and

 

Restrictions for QoS Support’’, on page 6-66.

 

 

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