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

Classify inbound traffic on these CoS types:

•IP-device (address)

•Protocol (LAN)

•VLAN-ID (VID).

•Source-Port

Apply 802.1p priority to selected traffic outbound on tagged VLANs.

Set Priority

Honor Priority

Downstream Switch

Tagged VLANs on inbound and outbound ports.

Traffic arrives with priority set by edge switch

Forward with 802.1p priority.

Downstream

Switch

Tagged VLANs on some or all inbound and outbound ports.

Classify inbound traffic on CoS types.

Change priority on selected CoS type(s).

Forward with 802.1p priority.

Change Priority

Honor New Priority

Downstream

Switch

Tagged VLANs on at least some inbound ports.

Traffic arrives with the priority set in the VLAN tag. Carry priority downstream on tagged VLANs.

Figure 15-1. Example of 802.1p Prioritization Based on CoS Types and Use of Tagged VLANs

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 DSCP markers to selected traffic.

Change Policy

Honor New Policy

Downstream

Switch

Classify on ToS Diffserv

Figure 15-2. 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.

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