Rule: There must be only one message path between any two BACnet devices on an internetwork. No communication loops are allowed.

Example: Do not configure multiple BACnet devices on the same LAN with both BACnet/IP and BACnet/Ethernet enabled. This includes host (remote controller) stations.

Rule: A BACnet router must exist between two BACnet networks (different network numbers) for BACnet messages to pass between the devices on the two networks. This applies to any link-layer types.

The important concept here is that a BACnet router can be a single-purpose device, or an application layer device (controller) that also includes router functionality, such as a station running in a remote host.

The BACnet Specification requires that all devices communicating on the BACnet network have an application layer entity, including a BACnet Device object.

By default, if a station is configured for multiple link-layers, it automatically acts as a router between two (or more) BACnet networks to which it is directly attached. However, a station does not act as a plain IP router.

For example, if an EthernetPort and two MstpPorts are enabled in a remote station, the station operates as a BACnet router between the three networks. If Ethernet, IP, and a MS/TP ports are enabled, the station operates as a BACnet-to-BACnet/IP router between the three networks, and so forth. If you add a second IpPort, bound to a different UDP port, the station routes BACnet messages among the networks, just like other data link layers.

Rule: Within any given internetwork, each BACnet network must have a unique network number, from 1 to 65534.

The concept is that each link-layer (network port), which is enabled in the BACnet network’s Bacnet Comm > Network container corresponds to a specific BACnet network, specified by the Network Number property.

If you establish a new BACnet internetwork, you can specify any network number for each port. For example, BACnet/IP (IpPort) = 1, BACnet/Ethernet (EthernetPort) = 2, and (if a QNX-based host), MstpPort (1) = 3, MstpPort (2) = 4, and so on.

If you add a host on an existing BACnet internetwork, you should specify the established BACnet/IP network number and/or BACnet/Ethernet network number currently in use. If you have a host with one or more enabled MstpPorts, you must also specify a previously-unused network number for each network.

Rule: Every BACnet object, including the Device object in each BACnet device, must have a unique numeric Object_Identifier.

In the specific case of the Device object, this Object_Identifier must be unique, internetwork-wide. Valid values for this identifier range from 0 to 4194302.

The concept is that BACnet devices respond to a Who-Is broadcast message with an I-Am reply that includes each Device object’s unique numeric Object_Identifier. If you receive duplicate I-Am messages (multiples showing the same device, by number), either more than one device has been assigned the same identifier (number) or a message loop exists.