# 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 NGFW Engines, Master NGFW Engines, and Virtual NGFW Engines in the Firewall/VPN role support QoS.

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

## Steps

1. Select Configuration.
2. Browse to Policies > Firewall Policies.
3. Right-click the Firewall Policy, then select Edit Firewall 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.