Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Table 6-4. Switch Classifier Search Order and Precedence

Search Order

Precedence

QoS Classifier

1

6

(lowest)

Incoming 802.1p Priority (present in tagged VLAN environments)

2

5

 

Incoming source-port on the switch

3

4

 

VLAN Priority

4

3

 

IP Type of Service (ToS) field (IP packets only)

5

2

 

Device Priority (destination or source IP address)

6

1

(highest)

UDP/TCP Application Type (port)

 

 

 

 

 

The switches use the lowest-to-highest search order shown in table 6-4to

 

identify the highest-precedence classifier to apply to any given packet. If there

 

is only one configured classifier that matches a given packet, then the switch

 

applies the QoS policy specified in that classifier. If multiple configured

 

classifiers match a given packet, the switch applies each one in turn to the

 

packet and concludes with the QoS policy for the highest-precedence

 

classifier. Note that if the highest precedence classifier is configured to apply

 

a DSCP policy, then both the DSCP in the packet and the 802.1p priority

 

applied to the packet can be changed. However, if the highest precedence

 

classifier is configured to apply an 802.1p priority rule, only the 802.1p priority

 

in the final QoS match for the packet is changed.

 

 

N o t e

On switches covered in this guide, intermixing lower-precedence classifiers

 

configured with DSCP policies and higher-precedence classifiers configured

 

with 802.1p priority rules is not recommended, as this can result in a packet

 

with an 802.1p priority assigned by one classifier and a DSCP policy by another

 

classifier. This is because the search order would allow a lower precedence

 

classifier configured with a DSCP policy to change both the DSCP and the

 

802.1p setting in a packet, and then would allow a subsequent, higher

 

precedence classifier configured with an 802.1p priority rule to change only

 

the 802.1p setting. To avoid this problem, a DSCP policy option should be

 

applied only on the highest-precedence classifier in use on the switch or

 

applied to all QoS classifiers in use on the switch.

 

In general, the precedence of QoS classifiers should be considered when

 

 

configuring QoS policies. For example, suppose that a system administrator

 

has used an 802.1p priority rule to assign a high priority for packets received

 

on VLAN 100, but has also used another 802.1p priority rule to assign a normal

 

priority for TCP port 80 packets received on the switch. Since TCP/UDP port

 

precedence supersedes VLAN precedence, all TCP port 80 packets on VLAN

 

100 will be set to normal priority. For a classifier precedence listing, see table

 

6-4, ‘‘Switch Classifier Search Order and Precedence’’, on page 6-10.

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