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. By

queue

default, there are eight outbound queues for each port in the switch. Queue 8 is the highest priority

 

queue; queue 1 is the lowest priority queue. Traffic in a port’s high priority queue leaves the switch

 

before any traffic in the port’s medium or low priority queues.

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: Each switch port has four shared outbound traffic queues; the queue with a priority value of one has the lowest priority, and priority value four has the highest priority. Packets leave the switch port on the basis of their queue assignment and whether any higher queues are empty:

Table 5-1. Port Queue Exit Priorities

Port Queue and

Priority for Exiting

802.1p Priority Values

From the Port

 

 

Low (1)

Fourth

Low (2)

 

Normal (0)

Third

Normal (3)

 

Medium (4)

Second

Medium (5)

 

High (6)

First

High (7)

 

 

 

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