# Use QoS Class elements to apply custom link selection options to traffic

You can use QoS Class elements in Access rules to override the default link selection options in Network Application elements and Protocol elements.

## Before you begin

You must create QoS Class elements and define link selection options in them.

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Note: Only layer 3 physical interfaces on Security Engines, Master Engines, and Virtual Engines in the Engine/VPN role support QoS.

For more details about the product and how to configure features, click Help or press F1.

## Steps

1. Select Engine.
2. Browse to Policies > Engine Policies.
3. Right-click the Engine Policy, then select Edit Engine Policy.
4. On the IPv4 Access tab or to the IPv6 Access tab, add a rule.
5. Define the source and destination according to your needs, or set the value to ANY.
6. In the Service cell, add elements that represent the type of traffic to which you want to apply custom link selection options.
   - To override the default link selection options in a Network Application element, add the Network Application element.
   - To override the default link selection options in a Protocol element, add a Service element that uses the Protocol.
7. Set the action to Allow or Continue.
8. In the QoS Class cell, add the QoS Class element that defines the link selection options.
9. Save and refresh the policy to transfer the changes.