BBMD

BBMD - BACnet Broadcast Management Device

In the "Old Days" all BBMDs had to have identical BBMD Distribution Tables. (BDTs), and only one active BBMD per IP subnet.

Now you are allowed multiple active BBMDs per subnet, but the BDTs must not have any identical peer IP entries (to avoid broadcast loops). ( J.4.3. "BBMD Concept", paragraph 2, of the spec )

So far so good. For full coverage of a given network, all subnets need a BBMD (or multiple) that point to every other BBMD in the network, so broadcasts can percolate from any subnet to all other subnets.

BUT the specification allows you to deliberately leave out connections to avoid forwarding broadcasts where they are not required. ( J.4.3. "BBMD Concept", paragraph 2, of the spec )