Adding an FD node in a standard topology
When speaking about BACnet IP we are speaking about a protocol running over UDP that has its own layer-based architecture (Data-Link, network, application). The BACnet/IP Data-link layer is the IP's UDP layer and its lower layers.
A BACnet IP network can be distributed in multiples IP networks (e.g. 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24). However, IP routing rules take precedence over the BACnet/IP datagrams. This is not different from any other protocol that runs over TPC or UDP.
The BACnet broadcast messages, used for many things, are IP broadcast messages. And thus, cannot travel between IP sub-networks. This is where the BACnet Broadcast Manager Device (BBMD) enters the game as shown in image 1.1.
Image 1.1: A single BACnet/IP network over two IP networks
In the previous image, there are six BACnet IP nodes divided into two different IP networks. All the six are under the same BACnet Network. The BBMD Z and Y must be present on each sub-network for all the BACnet nodes to communicate properly.
The BBMD could be seen as "switches" with tables (Broadcast Distribution Table or BDT) that only picks IP broadcasts messages with BACnet datagrams. In the previous example, the BBMD Z has a table with Y while BBMD Y does the same for Z.
Let's consider node A sends a Who-Is for everyone (Such as our BACnet client when doing a Discovery). There will be three IP datagrams as shown at Image 1.2:
The original A's IP Broadcast message over the 192.168.1.0/24 sub-network.
An IP unicast between Z and Y.
An IP broadcast over the 192.168.2.0/24 subnetwork send by Y.
The first and third steps are Broadcast messages which will propagate only inside the Broadcast domain. The second step will only have success if Z’s unicasts can be routed to Y. Normally,this falls under the IP routers jurisdiction.
After the Who-Is message arrives at the nodes D to F, they will answer with their I-Am broadcast messages. Therefore, the process repeats itself in reverse order for each one of those answers.
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