Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Precedence

Criteria

Overview

 

 

 

 

 

6

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

 

 

 

 

 

7

Incoming

Where a packet enters the switch on a tagged VLAN, if QoS is not configured to override the

 

802.1p

packet’s priority setting, the switch uses the packet’s existing 802.1p priority (assigned by an

 

Priority

upstream device or application) to determine which inbound and outbound port queue to use.

 

 

If the packet leaves the switch on a tagged VLAN, then there is no change to its 802.1p priority

 

 

setting. If the packet leaves the switch on an untagged 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 on an untagged VLAN, but exits on a tagged VLAN, then a tagged VLAN field, including an 802.1p priority of 0 (normal), is added to the packet.

No Override. By default, the IP ToS, Protocol, and VLAN-ID criteria auto­ matically list each of their priority options as No-override. (Some IP TOS codepoints use default priority settings defined by the DSCP standard.) This means that if you do not configure a priority for a specific option, QoS does not prioritize packets to which that option applies. For example, if you do not specify a priority for the IP protocol, then the IP protocol will not be a criteria for setting a QoS priority and the packets will be handled as described above.

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