You can use the FORWARD parameter to forward the specified network route exclusively over a specified interface.
If the specified interface is inactive, RouteD takes no special action to forward this network.
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.