D-Link DFL-1600 User Manual page 100

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4. Create interface group and add rules.
To be able to write rules with destination interface that can use either
route, you have to create a interface group that use the Security/Transport
Equivalent feature. This makes the two interfaces equal in a security sense.
Creating the interface group.
Interfaces Interface Groups
Enter the following:
Name: Type a name for the interface group.
Security/Transport Equivalent: Enable
Interfaces: Select the WAN1 and WAN2 interface.
Then click OK
Add rules.
Add rules using the newly created interface group as destination interface.
See 14.3 IP Rules Configuration
The default route for interface WAN2 will not be monitored. The reason
for this is that we have no backup route for the route over interface WAN2.
for details on how to configure rules.
Note
D-Link Firewalls User's Guide
Chapter 10. Routing
Add Interface Group: