Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Table 6-5.Precedence Criteria for QoS Classifiers

Precedence

Criteria

Overview

 

 

 

1

UDP/TCP

Takes precedence based on a layer 4 UDP or TCP application, with a user-specified application

 

 

port number (for example, Telnet). Default state: Disabled

 

If a packet does not meet the criteria for UDP/TCP priority, then precedence defaults to the Device Priority

 

classifier, below.

 

 

 

2

Device

Takes precedence based on an inbound packet having a particular destination or source IP

 

Priority

address. If a given packet has a destination IP address matching a QoS configuration, this

 

(IP

packet takes precedence over another packet that has the matching IP address as a source

 

Address)

address. (This can occur, for example, on an outbound port in a switch mesh environment.)

 

 

Also, if the source and destination IP addresses (SA and DA) in the same packet match for

 

 

different QoS policies, the DA takes precedence. Default state: No IP address prioritization.

 

If a packet does not meet the criteria for device priority, then precedence defaults to the IP Type of Service

 

(ToS) classifier, below.

 

 

 

3

IP Type-

Takes precedence based on the TOS field in IP packets. (Applies only to IP packets.) The ToS

 

of-

field is configured by an upstream device or application before the packet enters the switch.

 

Service

• IP Precedence Mode: QoS reads an inbound packet’s IP precedence (upper three) bits in

 

(IP ToS)

the Type-of-Service (ToS) byte and automatically assigns an 802.1p priority to the packet (if

specified in the QoS configuration) for outbound transmission.

•Differentiated Services (Diffserve) Mode: QoS reads an inbound IP packet’s differentiated services, or codepoint (upper six), bits of the Type-of-Service (TOS) byte. Packet prioritization depends on the configured priority for the codepoint. (Some codepoints default to the DSCP standard, but can be overridden.)

For more on IP ToS, see “QoS IP Type-of-Service (ToS) Policy and Priority” on page 6-34.

Default state: Disabled.

 

If a packet does not meet the criteria for ToS priority, then precedence defaults to the VLAN classifier.

4

VLAN

Takes precedence based on the ID number of the VLAN in which the inbound packet exists.

 

Priority

For example, if the default VLAN (VID = 1) and the “Blue” VLAN (with a VID of 20) are both

 

 

assigned to a port, and Blue VLAN traffic is more important, you can configure QoS to give Blue

 

 

VLAN traffic a higher priority than default VLAN traffic. (Priority is applied on the outbound

 

 

port.) Default state: No-override.

 

If a packet does not meet the criteria for VLAN priority, then precedence defaults to the Source-Port

 

classifier, below.

 

 

 

5

Source-

Takes precedence based on the source-port (that is, the port on which the packet entered the

 

Port

switch).

If a packet does not meet the criteria for source-port priority, then precedence defaults to Incoming 802.1p criteria, below.

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