Use of BBMDs (BACnet broadcast management device) is the answer to a BACnet/IP network that needs to span across multiple IP subnets. Each IP subnet with BACnet/IP devices requires one (and only one) BBMD.
A BBMD may be a device operating solely as a BBMD, or more typically, include BBMD functions in addition to other application/controller duties. This is the case with a NiagaraAX station running a Bacnet network with its IpPort, Link container’s “Ip Device Type” property set to “Bbmd” (instead of the default “Standard”, or “Foreign Device”).
Often an installation with BBMDs is not an “internetwork,” but have only one BACnet/IP network. If so, Niagara stations on
different subnets should specify the same network number for their associated IpPort.
A BBMD is used to support delivery of BACnet broadcast messages, such as “Who-Is” and “Who-Has.” As a rule, globally broadcast messages are inherently blocked by standard IP routers, which are used to connect separate IP subnets. Note this issue does not exist for any “directed message” between devices on different subnets, such as a common ReadProperty request—which is not a broadcast message.
The BBMD resolves this issue by acting as a broadcast manager for its subnet, working in coordination with other “peer” BBMDs. Each other IP subnet that contains BACnet/IP devices has one BBMD. Each BBMD stores a table with the IP address and distribution mask of all BBMDs, itself included. This is called the BBMD’s “BDT” (Broadcast Distribution Table), and is identical[2] in each BBMD for that entire BACnet/IP network.
When a global broadcast message is sent, it is automatically received by all devices on its local subnet, including the BBMD. (Local devices reply as needed to the broadcast device, without BBMD involvement.) The local BBMD forwards the broadcast message to the other subnets, using one of two methods (as defined for each remote subnet):
The BBMD sends the message using a “directed broadcast,” whereby the IP router for the destination subnet broadcasts the message on its local subnet. The router must support directed broadcasts, otherwise the Two-Hop method must be used. One-hop is unusual, as IP routers rarely pass directed broadcasts off or onto the local subnet.
The BBMD sends the message to its peer BBMD on the remote subnet, whereby that BMMD broadcasts the message on its local subnet.
Note that replies to BACnet broadcast messages may or may not require BBMD involvement. Occasionally, these are directed messages back to the particular BACnet/IP device that generated the message. However, replies to a “Who-Is” broadcast are often broadcast “I-Am” messages, as is the case with Niagara.
[2] The BDT in each BBMD is required to be identical in the original specification. However, Addendum 135-2008o (not currently implemented in Niagara) relaxes this requirement, in order to allow BACnet/IP communication in the Network Address Translation (NAT) environment.