Dual WAN with one as standby backup
From DD-WRT Wiki
Dual WAN with one as standby backup
This tutorial explains how you can assign one (or more) of the LAN ports as an extra WAN port. There is another tutorial Dual-WAN_for_simple_round-robin_load_equalization which explains how you can load balance between the two WAN connections. In this case I did not want to load balance; but rather create a standby WAN connection.
Create an extra VLAN
First you need to create an extra VLAN. In this case we want to to remove the port 4 from vlan0 and add it to the new vlan2
nvram set vlan0ports="1 2 3 5*"
nvram set vlan2ports="4 5"
nvram set vlan2hwname=et0
nvram set wan2_ifnames=vlan2
nvram set wan2_ifname=vlan2
nvram set wan2_mtu=1500
nvram commit
You can confirm the setting using the dd-wrt web interface