Contemporary Control Systems EIPR Wired/Wireless VPN Router Application Guide - Manual (Page 8)

AG-EIPR0000-BC1 Page 8
Application Guide — EIPR Series
When attaching BACnet devices to IP networks it is
possible that the IP network has been sub-netted
through the use of IP routers. Most IP routers will not
pass broadcast messages which are crucial to
BACnet’s operation. The solution is to incorporate
BACnet/IP Broadcast Management Device (BBMD)
functionality within the BACnet internetwork.
The BBMD concept 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 would appear 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 to be located
on one of the subnets with FDR devices registering to
this one BBMD. This is what is shown in the example
with a BAS Router providing BBMD functionality while
allowing for foreign devices registration. Notice that
connecting to a BACnet MS/TP network is an option.
Application #3 — Cascaded Routers for Additional Isolation
For increased security and isolation, IP routers can be
cascaded. Make sure that each LAN-side subnet
address is unique when cascading IP routers. The
left-most IP router can have its WAN-side IP address
assigned using DHCP client or by using static IP
address assignment.
The illustration shows a pair of EIPR routers, but the
right-most router could also be some other type of
router — perhaps one already existing in the business
system — because the EIPR supports standard
Internet protocols.
Application #4 — Limiting BACnet Traffic