Contemporary Controls EIGR Series Application Manual page 8

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.
192.168.92.101
192.168.92.102
Application #4 – Limiting BACnet Traffic
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.
Route between
BACnet/IP and
BACnet MS/TP
Field
Controller
Field
Controller
STATUS PWR
H
WAN
L
1
2
LAN
3
4
192.168.92.1
BASRT-B
Field
Controller
BASRT-B
Field
Controller
Application Guide – EIGR Series
The illustration shows a pair of EIGR routers, but the
right-most router could also be some other type of router
— perhaps one already existing in the business system —
because the EIGR supports standard Internet protocols.
STATUS PWR
RESET RESET
H
WAN
L
1
2
LAN
3
4
192.168.93.1
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.
Internet
Gateway
STATUS PWR
(firewall)
RESET
H
WAN
L
1
2
LAN
3
4
EIGR
Route between
IP Subnets
Automation
Controller
8
192.168.93.101
192.168.93.102
Automation Automation
Controller Controller
Automation
Controller
AG-EIGR0000-AA0