Not sure I understand the question.
The BMC is the IPMI handler in the servers boards.
For most the Intel boards, the BMC has a dedicated NIC module option exclusively for the use of the BMC and IPMI network commands.
The BMC is also able to share the network connection of the with the mother board NIC 0 or NIc 1. The default is NIC 0
There are also some limitations on network configurations when using multiple Network connection to the BMC. (The can not be on the same subnet)
Add in card NIC are not wired to the BMC so can not access the BMC via IPMI commands.
Within the OS local mode, IPMI commands can access the BMC,
I am not familiar with the sj2000? What board does it use?