From DD-WRT Wiki
 Current Status
Since the /etc/network/interfaces file that OpenWRT has, doesn't exist in DD-WRT, we can't use the command
ifup doesn't actually configure the interface, but accesses the ifconfig and other commands.
There's an example interface file that we could probably save to /jffs/etc/network/interfaces and use the command
ifup -i /jffs/etc/network/interfaces commands
The OpenWRT interfaces file can be found here: http://openwrt.org/CentralNvramMaps/EtcNetworkInterfaces?highlight=(interfaces)