You can use the FORWARD.COND parameter to forward the specified network route exclusively over a specified interface.
If the specified interface is inactive, RouteD forwards the network over all of the other interfaces.
Specify a network as both an IP address and a mask or as both an IP address and a bit number. The bit number n indicates which bit in the 0 – 32 bits of the IP address (counting left to right beginning from 0) is the last bit of the network portion of the IP address. If the MASK and bit number are missing, a mask of 255.255.255.255 is used.