BBMD设置的目的就是让两个不同网段的设备可以同时在一个系统中访问的操作。 比如你有两个bacnet的网络，但是一个是192.168.0.1-192.168.0.255的网段，另一个是10.0.0.1-10.0.0.255的网段，那么你就需要设置BBMD才能使两个网段的 设备能相互访问。 不管在那个品牌的系统中，在大型的组网中，BBMD的设置都是必须的，可以实现全局控制器的控制，也便于复杂控制逻辑的实现。

BACnet/IP network

A BACnet/IP network is a collection of one or more IP subnetworks (IP domains) that are assigned a single BACnet network Number B/IP Concept A BACnet/IP network shall function in concept identically to the other non‐IP network types with respect to directed messages and broadcast messages, including local, remote, and global broadcasts, as defined in 6.3.2: a directed message shall be sent directly to the destination node; a "local broadcast" shall reach all nodes on a single B/IP network; a "remote broadcast" shall reach all nodes on a single BACnet network with network number different from that of the originator's network; a "global broadcast" shall reach all nodes on all networks. The management of broadcasts within a single B/IP network, or between multiple B/IP networks, or between B/IP and non‐B/IP networks, is described in J.4.

BACnet Virtual Link

This Annex specifies the BACnet Virtual Link Control (BVLC) functions required to support BACnet/IP directed and broadcast messages The BACnet Virtual Link Layer (BVLL) provides the interface between the BACnet Network Layer (Clause 6) and the underlying capabilities of a particular communication subsystem. This Annex specifies the BACnet Virtual Link Control (BVLC) functions required to support BACnet/IP directed and broadcast messages BBMD Concept Each IP subnet that is part of a B/IP network comprised of two or more subnets shall have one, and only one, BBMD. Each BBMD shall possess a table called a Broadcast Distribution Table (BDT) which shall be the same in every BBMD in a given B/IP network. If the BBMD has also been designated to register foreign devices as described below, it shall also possess a Foreign Device Table (FDT). 核心是一个 B/IP 网络内的广播问题（同一个网络内，网络号 相同；BVLC 工作在网络层一下）。

"two‐hop" distribution

There are two ways that a BBMD may distribute broadcast messages to remote IP subnets. The first is to use IP "directed broadcasts" (also called "one‐hop" distribution). This involves sending the message using a B/IP address in which the network portion of the address contains the subnet of the destination IP subnet and the host portion of the address contains all 1's.（IP 地址由网络地址、主机地址组成，主机域全为 1， 目的地址为一个子网）。 While this method of distribution is efficient, it requires that the IP router serving the destination subnet be configured to support the passage of such directed broadcasts. Since not all IP routers are configured to pass directed broadcasts, a BBMD may be configured to send a directed message to the BBMD on the remote subnet ("two‐hop" distribution) which then transmits it using the B/IP broadcast address. Since the use of one‐hop distribution requires an IP router configuration that may or may not be possible, while the two‐hop method is always available, the choice of which method to use in any given case is a local matter Broadcast Distribution Table Format The BDT consists of one entry for each BBMD within a B/IP network. Each entry consists of the 6‐octet B/IP address of the BBMD serving the IP subnet and a 4‐octet broadcast distribution mask.

广播散步掩码的两种情况：（不一定是子网掩码）

1、If messages are to be distributed on the remote IP subnet using directed broadcasts, the broadcast distribution mask shall be identical to the subnet mask associated with the subnet, i.e., all 1's in the network portion of the 4‐octet IP address field and all 0's in the host portion.（1 跳） 2、If messages are to be distributed on the remote IP subnet by sending the message directly to the remote BBMD, the broadcast distribution mask shall be all 1's. The use of the broadcast distribution mask is described in J.4.5.（2 跳） BBMD Operation ‐ Broadcast Distribution 1、配置 BDT 表的有两种方式，当然通过本地方式配置是最简单的了，通过配置文件就 可以完成功能了。 Upon receipt of a BVLL Write‐Broadcast‐Distribution‐Table message, a BBMD shall attempt to create or replace its BDT, depending on whether or not a BDT has previously existed. If the creation or replacement of the BDT is successful, the BBMD shall return a BVLC‐Result message to the originating device with a result code of X'0000'. Otherwise, the BBMD shall return a BVLC‐Result message to the originating device with a result code of X'0010' indicating that the write attempt has failed. Upon receipt of a BVLL Read‐Broadcast‐Distribution‐Table message, a BBMD shall load the contents of its BDT into a BVLL Read‐Broadcast‐Distribution‐Table‐Ack message and send it to the originating device. If the BBMD is unable to perform the read of its BDT, it shall return a BVLC‐Result message to the originating device with a result code of X'0020' indicating that the read attempt has failed.

2、Original‐Broadcast‐NPDU:普通设备广播消息格式

一、源 BBMD

Upon receipt of a BVLL Original‐Broadcast‐NPDU message, a BBMD shall construct a BVLL Forwarded‐NPDU message and send it to each IP subnet in its BDT with the exception of its own. The B/IP address to which the Forwarded‐NPDU message is sent is formed by inverting（取反） the broadcast distribution mask in the BDT entry and logically Oring it with the BBMD address of the same entry. This process produces either the directed broadcast address of the remote subnet or the unicast address of the BBMD on that subnet depending on the contents of the broadcast distribution mask. See J.4.3.2..    In addition, the received BACnet NPDU shall be sent directly to each foreign device currently in the BBMD's FDT also using the BVLL Forwarded‐  NPDU message.

二．目的 BBMD

Upon receipt of a BVLL Forwarded‐NPDU message, a BBMD shall process it according to whether it was received from a peer BBMD as the result of a directed broadcast or a unicast transmission.（有个判断过程）    A BBMD may ascertain the method by which Forwarded‐  NPDU messages will arrive by inspecting the broadcast distribution mask field in its own BDT entry since all BDTs are required to be identical. If the message arrived via directed broadcast, it was also received by the other devices on the BBMD's subnet. In this case the BBMD merely retransmits the message directly to each foreign device currently in the BBMD's FDT. （FDT 不一定样了） If the message arrived via a unicast transmission it has not yet been received by the other devices on the BBMD's subnet. In this case, the message is sent to the devices on the BBMD's subnet using the B/IP broadcast address as well as to each foreign device currently in the BBMD's FDT. A BBMD on a subnet with no other BACnet devices may omit the broadcast using the B/IP broadcast address. The method by which a BBMD determines whether or not other BACnet devices are present is a local matter.（如果没有设备当然没有必要广播了，判断依据不在标准 之内）。

三、外部设备来的消息处理

Upon receipt of a BVLL Distribute‐Broadcast‐To‐Network message from a foreign device, the receiving BBMD shall transmit a BVLL Forwarded‐NPDU message on its local IP subnet using the local B/IP broadcast address as the destination address. In addition, a Forwarded‐NPDU message shall be sent to each entry in its BDT as described above in the case of the receipt of a BVLL Original‐Broadcast‐NPDU as well as directly to each foreign device currently in the BBMD's FDT except the originating node. If the BBMD is unable to perform the forwarding function, it shall return a BVLC‐Result message to the foreign device with a result code of X'0060' indicating that the forwarding attempt was unsuccessful. 整理：jimwit