Internetwork Rules

When implementing a BACnet internetwork there are several rules that must be followed:

Rule: There must be only one message path between any two BACnet devices on an internetwork. No communication “loops” are allowed.
- Example: You should not configure multiple BACnet devices on the same LAN with both BACnet/IP and BACnet/Ethernet enabled—note this includes Niagara stations (hosts) as well.
Rule: A BACnet router must exist between two different BACnet networks (different network numbers) for BACnet messages to pass between the devices on the two networks. This applies to any link-layer types.
- Concept: 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 Niagara host.
  With recent changes to the BACnet Specification, all devices communicating on the BACnet network must have an application layer entity, including a BACnet Device object.
  
  By default, if a Niagara station is configured for multiple link-layers, it automatically acts as a router between those 2 (or more) BACnet networks to which it is directly attached.
  
  For example, if an EthernetPort and 2 MstpPorts are all enabled in a JACE station, it operates as a BACnet router between those 3 networks. If Ethernet, IP, and a MSTP port are enabled, the station operates as a BACnet-to-BACnet/IP router between those 3 networks, and so forth. Note that if a second IpPort is added, bound to a different UDP port, the station routes BACnet messages between those networks, just like other data link layers.
  
  However, note that a Niagara station does not act as a “plain IP router”.
Rule: Within any given internetwork, each BACnet network must have unique network number, from 1 to 65534.
- Concept: Each link-layer (network port) that is enabled in the Bacnet network’s BacnetComm, Network container corresponds to a specific BACnet network, specified in the “Network Number” property.
  - If establishing a new BACnet internetwork, you can specify whatever network number for each port. For example, BACnet/IP (IpPort) = 1, BACnet/Ethernet (EthernetPort) = 2, and (if a QNX-based JACE), MstpPort (1) = 3, MstpPort (2) = 4, and so on.
  - If adding a JACE on an existing BACnet internetwork, you should specify the established BACnet/IP network number and/or BACnet/Ethernet network number currently in use. If a JACE with one or more enabled MstpPorts, you must also specify a previously unused network number for each one.
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.
- Concept: BACnet devices respond to a Who-Is broadcast messge with an I-am message that includes each Device object’s unique numeric Object_Identifier. If you receive duplicate I-am messages (multiples showing same device, by number), it means either that more than one device has been assigned that same identifier (number), or that a message loop exists.