I've a somewhat dated, yet still relevant document on the Sideband interface available here: Intel® Sideband Technology: Server Manageability Interfaces.
We provide some common filters that are built in, such as the IPMI port. However you can filter on anything you like, including a dedicated MAC address, which would allow your BMC to do anything it likes. You can filter on any port, ethertype,vlan tag etc that you wish, is fully configurable.
The common filters are there for legacy reasons, from back when the BMC and the host always shared a MAC address; while this design is not used as much as it was 10 years ago, it is still a valid usage model and as such we continue to maintain those build-in capabilities.
I hope this helps.