User Manual

Network architecture
BACnet architecture (MLN & ALN)
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IPv6 (BVLLv6) only supports two-hop BBMD. Broadcasts are implemented via
IPv6 mutlicasts. See
Ethernet, TCP/IP, MS/TP and BACnet basics (CM110666).
BBMDs ensure that broadcast messages are distributed in a BACnet network.
They are grouped by BACnet network. A maximum of one BBMD is allowed in
any one IP segment.
BACnet network #100 is separated by IP routers. The Internet also contains IP
routers. This is why different segments are shown before and after the Internet
cloud. BBMDs are required so that BACnet broadcast messages are available in
all IP segments.
The BBMD parameters are written to the BBMD or (for Desigo) to the BACnet
router during commissioning. The following information is required for each
BBMD in the BACnet network:
Designation Description
IP address IP address of the BBMD.
UDP port UDP port number of the BBMD.
Broadcast mask If the BBMD is to be addressed via direct broadcast (one-hop distribution) the subnet mask of
the BBMD must be specified. Since not all IP routers support this mechanism, direct
broadcasts are not supported by default. Two-hop-distribution is always possible. The
broadcast mask is then 255.255.255.255.
Not required for IPv6.
A foreign device is a (remote) BACnet device in a remote IP segment. It
registers with a BBMD in order to send or receive broadcast messages.
Registration with a BBMD involves making an entry in its Foreign Device Table
(FDT). The registration must be renewed at regular intervals.
The foreign device does not send broadcast messages, but passes them as
unicast messages to the BBMD for distribution. The BBMD in turn passes
incoming broadcast messages as unicast messages to the foreign devices in its
FDT.
In the Desigo system, Desigo CC, XWP/ABT, and PXM30/40/50-E can be
operated as foreign devices. IPv6 does not support foreign devices
● IP Segment 1: Desigo CC 1 does not have to be configured as a foreign
device, because this IP segment contains a BBMD.
● IP Segment 2: Desigo CC 3 does not have to be configured as foreign
device, because this IP segment contains a BBMD.
● IP Segment 3: This segment only contains Desigo CC 2. To enable Desigo
CC 2 to receive and send broadcast messages, it must register with a
BBMD as a foreign device. It does not matter with which BBMD it registers.
If a BACnet device operates as a foreign device, the IP address and UDP port
number of the BBMD must be specified.
Designation Description
IP Address of BBMD IP address of the BBMD with which the foreign device registers.
UDP Port of BBMD UDP port number of the BBMD with which the foreign device is registered. The default is
0xBAC0.
The recording interval (Time-To-Live) for Desigo products is set at 300 seconds
(= 5 minutes).
PTP data link layer
The PTP Data Link Layer is used for remote management over the telephone
line. Unlike LonTalk and IP, PTP does not allow the creation of a network. The
PTP connection is always between two half routers, and between two different
BACnet networks. Several BACnet networks may be located at each end of the
PTP connection. Only one active communication line can exist between any two
BACnet networks or between any BACnet devices.
BBMD parameters
Foreign device
Examples from the
Desigo topology
Foreign device
parameters