BBMD

BBMD - BACnet Broadcast Management Device

Broadcasts are usually blocked by IP routers. In this case, discovery mechanisms such as Who-Is broadcasts cannot reach devices on other IP Subnets. This restriction is overcome by placing a BBMD in each of the IP Subnets.

BBMDs are devices (usually 'virtual', it, part of another BACnet Device of Router) that are responsible for receiving a broadcast on an IP Subnet, transferring it using a unicast other BBMDs on other IP Subnets, which then rebroadcast on those IP Subnets.

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 )