Issue
Configuration of multiple BACnet BBMD's over several subnets
Product Line
Andover Continuum, EcoStruxure Building Operation
Environment
Bacnet BBMD
Cause
Bacnet Broadcast communication through IP router
Resolution
Configuration of multiple BBMD's over several subnets:
INCORRECT METHOD
For example if there are 4 subnets on the same B/IP (BACnet IP Network):
- Subnet 1: Workstation setup as a BBMD with an entry in the Broadcast Distribution Table for each BCX
- Subnet 2: BCX setup as a BBMD with an entry in the Broadcast Distribution Table for itself and the Workstations BBMD Only.
- Subnet 3: BCX setup as a BBMD with an entry in the Broadcast Distribution Table for itself and the Workstations BBMD Only.
- Subnet 4: BCX setup as a BBMD with an entry in the Broadcast Distribution Table for itself and the Workstations BBMD Only.
The BCX's do not reference any points from each other. Since there is no data sharing between the BBMD's, one might think that in each BCX's Broadcast Distribution Table that there would only be a need to add one entry for the Workstations BBMD. Sounds like with this setup we'll reduce traffic by only adding the one entry in each BCX's BBMD Broadcast Distribution Table.
The above scenario could in fact slow down a network and cause unnecessary traffic.
The supported method is that every BBMD on a B/IP network have the same routing tables(better known as the Broadcast Distribution Table) as in the below configuration.
CORRECT METHOD
For example there are 4 subnets on the same B/IP (BACnet IP Network).
- Subnet 1: WS BBMD with an entry in the Broadcast Distribution Table for itself and all BCX's setup as BBMD on the same B/IP.
- Subnet 2: BBMD with an entry for all BBMD's(including self) in the Broadcast Distribution Table on the same B/IP.
- Subnet 3: BBMD with an entry for all BBMD's(including self) in the Broadcast Distribution Table on the same B/IP.
- Subnet 4: BBMD with an entry for all BBMD's(including self) in the Broadcast Distribution Table on the same B/IP.
The entry below can be found in the BACnet Standards 2004:
J.4.3 BBMD Concept
Each IP subnet that is part of a B/IP network comprised of two or more subnets shall have one, and only one, BBMD. Each BBMD shall possess a table called a Broadcast Distribution Table (BDT) which shall be the same in every BBMD in a given B/IP network.
