[ixp1200] Does anyone know how to enable ip_forwarding function of BV Linux?

Hi, all I'm testing some program on SDK2.0, which has Linux running on StrongARM(the kernel version is 2.3.99-pre3-rmk3-np2). I wonder how to enable ip_forwarding on that version. It was said that you can use "echo 1 > /proc/sys/net/ipv4/ip_forwarding" to enable it on recent version, but I can't find that file on StrongARM. Thanks Victor