Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Term

Use in This Document

 

 

outbound port

For any port, a buffer that holds outbound traffic until it can leave the switch through that port. There

queue

are four outbound queues for each port in the switch: high, medium, normal, and low. Traffic in a port’s

 

high priority queue leaves the switch before any traffic in the port’s medium priority queue, and so-on.

re-marking

Assigns a new QoS policy to an outbound packet by changing the DSCP bit settings in the ToS byte.

(DSCP re-

 

marking)

 

tagged port

Identifies a port as belonging to a specific VLAN and enables VLAN-tagged packets belonging to that

membership

VLAN to carry an 802.1p priority setting when outbound from that port. Where a port is an untagged

 

member of a VLAN, outbound packets belonging to that VLAN do not carry an 802.1p priority setting.

Type-of-Service

Comprised of a three-bit (high-order) precedence field and a five-bit (low-order) Type-of-Service field.

(ToS) byte

Later implementations may use this byte as a six-bit (high-order) Differentiated Services field and a

 

two-bit (low-order) reserved field. See also “IP-precedence bits” and DSCP elsewhere in this table.

upstream

A device linked directly or indirectly to an inbound switch port. That is, the switch receives traffic from

device

upstream devices.

 

 

Overview

QoS settings operate on two levels:

■Controlling the priority of outbound packets moving through the switch: Depending on the Qos Pass-Through mode setting, each switch port has up to four outbound traffic queues; “low”, “normal”, “medium”, and “high” priority. Packets leave the switch port on the basis of their queue assignment and whether any higher-priority queues are empty:

Table 6-1. Port Queue Exit Priorities

Port Queue and

Priority for Exiting

802.1p Priority Values

From the Port

 

 

Low (1 - 2)

Fourth

Normal (0, 3)

Third

Medium (4 - 5)

Second

High (6 - 7)

First

 

 

A QoS configuration enables you to set the outbound priority queue to which a packet is sent. (In an 802.1Q VLAN environment with VLAN- tagged ports, 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.)

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