Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

Quality of Service is a general term for classifying and prioritizing traffic throughout a network. That is, QoS enables you to establish an end-to-end traffic priority policy to improve control and throughput of important data. You can manage available bandwidth so that the most important traffic goes first. For example, you can use Quality of Service to:

■Upgrade or downgrade traffic from various servers.

■Control the priority of traffic from dedicated VLANs or applications.

■Change the priorities of traffic from various segments of your network as your business needs change.

■Set priority policies in edge switches in your network to enable traffic- handling rules across the network.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edge Switch

 

 

 

Honor Priority

 

 

 

 

Downstream

 

 

 

 

Honor New Priority

 

Classify inbound traffic

 

 

 

 

 

 

 

Switch

 

 

 

 

 

 

Downstream

 

 

 

 

 

Downstream

 

on these Class-of-

 

 

Tagged VLANs on some

 

 

 

 

Service (CoS) types:

 

Switch

 

or all inbound and

 

 

 

Switch

 

• IP-device (address)

 

Tagged VLANs on

 

 

outbound ports.

 

 

 

Tagged VLANs on at

 

• VLAN-ID (VID).

 

 

 

 

 

 

 

 

 

 

 

inbound and outbound

 

Classify inbound traffic

 

 

 

least some inbound

 

• Source-Port

 

ports.

 

on CoS types.

 

 

 

ports.

 

Apply 802.1p priority to

 

Traffic arrives with

 

Change priority on

 

 

 

Traffic arrives with the

 

 

priority set by edge

 

 

selected CoS type(s).

 

 

 

priority set in the VLAN

 

selected outbound

 

 

 

 

 

 

 

switch

 

 

 

 

 

 

 

 

 

tag. Carry priority

 

traffic on tagged VLANs.

 

 

Forward with 802.1p

 

 

 

 

 

 

 

 

 

 

 

 

 

 

downstream on tagged

 

 

 

 

 

 

Forward with 802.1p

 

 

priority.

 

 

 

 

 

 

 

 

 

 

 

 

 

VLANs.

 

 

 

 

 

 

priority.

 

 

 

 

 

 

 

 

 

 

 

Set Priority

 

 

 

 

 

 

Change Priority

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6-1. Example of 802.1p Priority Based on CoS (Class-of-Service) Types and Use of VLAN Tags

Edge Switch

Classify inbound traffic on IP-device (address) and VLAN-ID (VID).

Apply DSCP markers to selected traffic.

Set Policy
Honor Policy

Downstream

Switch

Traffic arrives with DSCP markers set by edge switch

Classify on ToS DiffServ.

Downstream

Switch

Classify on ToS DiffServ and Other CoS

Apply new DSCPmarkers to selected traffic.

Change Policy
Honor New Policy

Downstream

Switch

Classify on ToS Diffserv

Figure 6-2. Example Application of Differentiated Services Codepoint (DSCP) Policies

At the edge switch, QoS classifies certain traffic types and in some cases applies a DSCP policy. At the next hop (downstream switch) QoS honors the policies established at the edge switch. Further downstream, another switch may reclassify some traffic by applying new policies, and yet other downstream switches can be configured to honor the new policies.

6-4