Quality of Service (QoS): Managing Bandwidth More Effectively

Introduction

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 Class-of- Service (CoS) types:

IP-device (address)

VLAN-ID (VID).

Source-Port

Apply 802.1p priority to selected outbound traffic 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 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 DSCP markers 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.

QoS is implemented in the form of rules or policies that are configured on the switch. While you can use QoS to prioritize only the outbound traffic while it is moving through the switch, you derive the maximum benefit by using QoS

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