BACnet routing behavior in an IP-based Metasys system - Metasys - LIT-12012458 - Field Device - 13.0

Metasys IP Networks for BACnet/IP Controllers Technical Bulletin

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In an MS/TP based system, the scope of a BACnet broadcast is limited to the devices connected to the MS/TP trunk. This is the behavior which a network engine exhibits if BACnet Routing is disabled on the network engine . If BACnet Routing is enabled on the network engine , the network engine will forward all BACnet broadcasts it receives on one of its interfaces (an MS/TP trunk or its Ethernet port) out on all of its other interfaces (the other MS/TP trunk and/or its Ethernet port). When sent out on the Ethernet port, the BACnet broadcast will be sent as a local IP broadcast (that is addressed to the network engine ’s local subnet).

On an IP network, broadcast traffic typically doesn't travel between subnets. BACnet Broadcast Management Devices (BBMDs) are used to forward only BACnet broadcasts between subnets. If the network engine is configured as a BBMD, it will forward any BACnet broadcast it receives as a unicast message to all the other BBMDs in its BBMD table. When a network engine configured as a BBMD receives a forwarded BACnet broadcast, it sends the forwarded BACnet broadcast as a local IP broadcast. The network engine only sends the BACnet broadcast out over its MS/TP trunks if BACnet Routing is enabled.

If the IP controllers are in the same subnet as their supervising network engine and the network engine is configured as a BBMD, the IP controllers will receive all forwarded BACnet broadcasts received by their network engine . The effect is the same as when BACnet Routing is enabled on the network engine in an MS/TP based system.

The grouping of the network engine s together in the same VLAN or in a VLAN with the IP controllers they supervise is addressed in more detail in the following sections.