Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Overview

QoS settings operate on two levels:

�Controlling the priority of outbound packets moving through the switch: Each switch port has four outbound traffic queues; “low”, “nor­ mal”, “medium”, and “high” priority. Packets leave the switch port on the basis of their queue assignment and whether any higher queues are empty:

Table 15-1.Port Queue Exit Priorities

Port Queue

Priority for Exiting From the Port

High (6 - 7)

First

Medium (4 - 5)

Second

Normal (0, 3)

Third

Low (1 - 2)

Fourth

 

 

A QoS configuration enables you to set the outbound priority queue to which a packet is sent. (In an 802.1Q tagged VLAN environment, if QoS is not configured on the switch, but is configured on an upstream device, the priorities carried in the packets determine the forwarding queues in the switch.)

�Configuring a priority for outbound packets and a service (prior­ ity) policy for use by downstream devices:

•DSCP Policy: This feature enables you to set a priority policy in outbound IP packets. (You can configure downstream devices to read and use this policy.) This method is not dependent on tagged VLANs to carry priority policy to downstream devices, and can:

–Change the codepoint (the upper six bits) in the TOS byte.

–Set a new 802.1p priority for the packet.

(Setting DSCP policies requires IPv4 inbound packets. Refer to the “IPv4” entry under “Terminology” on page 15-5.)

•802.1p Priority: If an outbound packet is in an 802.1Q tagged VLAN environment (that is, if the packet is assigned to a tagged VLAN on the outbound port), then the packet carries an 802.1p priority setting that was configured in the switch. This priority setting ranges from 0 to 7, and can be used by downstream devices having up to eight outbound port queues. Thus, while packets within the switch move at the four priority levels shown in table 15-1,above, they still can carry an 802.1p priority that can be used by downstream devices having more or less than the four priority levels in the Series 5300XL switches. Also, if the packet enters the switch with an 802.1p priority setting, QoS can override this setting if configured to do so.

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