Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Packet Types and Evaluation Order

The switches covered by this chapter provide two QoS types (packet-matching criteria) you can use to configure QoS priority.

Table 6-4. Switch Type Search Order and Precedence

Search Order

Precedence

QoS type

1

2

Interface - Incoming source-port on the switch

2

1 (highest)

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

 

 

 

The switches use the lowest-to-highest search order shown in table 6-4to identify the highest-precedence type to apply to any given packet. If there is only one configured type that matches a given packet, then the switch applies the QoS policy specified in that type. If multiple configured types match a given packet, the switch applies each one in turn to the packet and concludes with the QoS policy for the highest-precedence type. Note that if the highest precedence type 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 type 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

Intermixing lower-precedence types configured with DSCP policies and

 

higher-precedence types configured with 802.1p priority rules is not

 

recommended, as this can result in a packet with an 802.1p priority assigned

 

by one type and a DSCP policy by another type. This is because the search

 

order would allow a lower precedence type 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 type 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 type in use on the switch

 

or applied to all QoS types in use on the switch.

 

 

6-9