In these boards, the ipmi interface can only be configured on the 1rst LAN or in the dedicated one, and you can choose it by issuing some raw commands.
My steps to enable the dedicated LAN on a X8DTU-F board are these:
#install ipmitool (this is for debian)
apt-get install ipmitool
#insert the kernel modules needed for ipmi
#get the current mode (01 00 is dedicated mode)
ipmitool raw 0x30 0x70 0x0c 0
#send the raw command to enable dedicated lan
ipmitool raw 0x30 0x70 0xc 1 1 0
You can now use ipmitool to assign an ipaddress/netmask to the interface, and start working with it normally.
If you want to know all the comands in raw mode for the supermicro ipmi, there's a list here