When connecting BACnet devices to IP networks, the IP network can be subnetted through the use of IP routers. As most IP routers will not pass broadcast messages, you can incorporate BACnet/IP Broadcast Management Device (BBMD) functionality within the BACnet internetwork.
BBMD requires that a broadcast message originating on one subnet be encapsulated into a directed message and sent to all remote subnets since these directed messages will pass through IP routers. Once the encapsulated messages are received on the remote subnets, a BBMD device will decode the message and resend it on its local subnet as a broadcast message. Therefore, it seems that a BBMD device must be present on each subnet in order to provide this encoding and decoding function. However, this is not the case if all the BACnet/IP devices support Foreign Device Registration (FDR). At a minimum, one BBMD device is required on one of the subnets, with FDR devices registering to this one BBMD. The following example illustrates this configuration, with a BASrouter providing BBMD functionality while allowing FDR.
Our BASrouter Application Guide provides BACnet routing examples that explain how our BASrouters can be used to solve a particular need. Application 8 in the Guide explains how to use routers to connect subnetted IP networks to a BACnet MS/TP network. The explanation shows how Contemporary Controls' Skorpion EIPR IP router connects the subnetted IP networks together and our BASRT-B BACnet router interconnects the BACnet/IP networks to the BACnet MS/TP network.
Not all IP Routers allow for their firewalls to be disabled, but this is a feature of the Skorpion EIPR routers. In this application, the EIPR IP Router firewall is enabled.
A firewall requires that message initiation begins on the LAN-side of the IP router and not from the WAN-side. Therefore, it would be impossible for a request initiated on the WAN side to reach the BASrouter located on the LAN side unless Port Forwarding is enabled on the IP router.
The BASrouter is webpage configurable. It has a number of troubleshooting/diagnostic capabilities and a BACnet MS/TP "Status" webpage that contains a graphical table of all of the MS/TP devices on the network and their status.
For a detailed explanation of Incorporating BACnet/IP Broadcast Management Device (BBMD) to Connect a BACnet MS/TP Network Subnetted IP Networks, read Application 8 in the BASrouter Application Guide (PDF).
