Cimetrics B3075 - Understanding the BACnet;IP Router

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What does a BACnet/IP to BACnet/IP router do, and why is it used?
Many BACnet-based building automation system (BAS) installations contain more than one BACnet
network. BACnet routers are used to selectively forward BACnet messages between BACnet networks,
allowing the creation of a BACnet internetwork. BACnet/IP employs the UDP protocol (IETF RFC 768) to
enable interoperable communication of BACnet messages over IP (Internet Protocol) networks, typically
using twisted-pair Ethernet as the data link technology. Since the release of its specification in 1999,
BACnet/IP has become increasingly popular even for relatively low-cost BAS devices.
In some facilities, BACnet/IP devices are connected to an IP network managed by the end customer’s IT
department to reduce the cost of BAS network construction and maintenance. However, there are
situations in which it is desirable to create multiple BACnet/IP networks; for example, the BAS integrator
might want to put most or all the BACnet/IP devices on one or more private IP networks (installed by the
integrator) but use an IT-managed IP network for communication between the private networks or between
buildings. The devices on the private network will have IP addresses that are assigned by the BAS
integrator. The Cimetrics B3075 was designed for this situation, as shown in the following diagram.
The B3075 has two Ethernet ports that are used for BACnet/IP communication, and each of those ports
will be assigned an IP address appropriate for the directly connected IP network. The B3075 is not an IP
router, so the IP addresses of devices connected to the backbone network will not appear on the private
network, and the IP addresses of devices connected to the private network will not appear on the
backbone network. BACnet/IP network packets originating from devices on the private network will
contain the B3075’s backbone network IP address when they are forwarded to the backbone network, and
BACnet/IP network packets originating from devices on the backbone network will contain the B3075’s
private network IP address on the private network. The IP address of the originating BACnet/IP device is
not lost when a message passes through a BACnet router; it is encoded within the BACnet NPDU by the
B3075 in accordance with the requirements of the BACnet Standard.