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Configuration of multiple BACnet BBMD's over several subnets

Issue

Configuration of multiple BACnet BBMD's over several subnets

Environment

Continuum

Bacnet BBMD

Cause

Bacnet Broadcast communication through IP router

Resolution

Configuration of multiple BBMD's over several subnets: BACnet TCP/IP Subnet Illustration.pdf

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.

Let's take a look at the BACnet standard's supported configuration on multiple BBMD's over several subnets.

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 which is the current standard

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.

 

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