Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Table 6-5. Precedence Criteria for QoS types

Precedence Criteria Overview

1IP Type- of- Service (IP ToS)

Takes precedence based on the TOS field in IP packets. (Applies only to IP packets.) The ToS field is configured by an upstream device or application before the packet enters the switch.

•IP Precedence Mode: QoS reads an inbound packet’s IP precedence (upper three) bits in 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 6-16.Default state: Disabled.

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

2

Interface

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

 

(Source-

switch).

 

 

 

Port)

 

 

 

 

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

 

criteria, below

 

 

 

 

 

3

Incoming

Where a VLAN-tagged packet enters the switch through a port that is a tagged member of that

 

802.1p

VLAN, if QoS is not configured to override the packet’s priority setting, the switch uses the

 

Priority

packet’s existing 802.1p priority (assigned by an upstream device or application) to determine

 

 

which inbound and outbound port queue to use. If there is no QoS policy match on the packet,

 

 

and it then leaves the switch through a port that is a tagged member of the VLAN, then there

 

 

is no change to its 802.1p priority setting. If the packet leaves the switch through a port that is

 

 

an untagged member of the VLAN, the 802.1p priority is dropped.

 

 

Entering

Outbound Port

Exiting

 

 

(Inbound) 802.1p

Queue

(Outbound)

 

 

Priority

 

802.1p Priority

 

 

1 - 2

Low

1 - 2

 

 

0 - 3

Normal

0 - 3

 

 

4 - 5

Medium

4 - 5

 

 

6 - 7

High

6 - 7

If a packet does not meet the criteria for Incoming 802.1p priority, then the packet goes to the “normal” outbound queue of the appropriate port. If the packet entered the switch through a port that is an untagged member of a VLAN, but exits through a VLAN-tagged port, then an 802.1Q field, including an 802.1p priority, is added to the packet header. If no QoS policy is configured or applied to the packet, then the 802.1p priority of 0 (normal) is assigned to the packet for outbound transmission.

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