For the Converged Network architecture, the BAS network is installed with much of its own infrastructure, however all IP addresses and VLANs are allocated from the IT network's address space. The IP addresses allocated by IT are assigned to the BAS devices by DHCP servers configured on the BAS network switches. All the BAS devices are physically connected to dedicated BAS network switches.
The converged architecture is applicable when the IT department is able to allocate all the IP addresses, but also wants a well-defined interface between the BAS network and the rest of the network. Other reasons for implementing the converged network would be if IT does not have the required switch ports available for the BAS equipment and is unable to purchase and deploy the additional switching infrastructure. The converged architecture also gives IT direct visibility to the IP controllers (for example, IT can ping them) while the segmented network architecture does not (the IP controllers are only visible through their supervising network engine ).
Figure 1 depicts a Converged BAS network. VLANs A and B each host a network engine and those BACnet/IP controllers that it supervises. By grouping the network engine and the supervised IP controllers in the same VLAN/subnetwork, they can be assigned IP addresses within the same, easily identifiable range. At the same time it creates a network equivalent to an MS/TP-based system with BACnet routing enabled on all the network engine s. That is, a BACnet broadcast initiated by any network engine or IP controller will be seen by all other network engine s and IP controllers. Because each network engine is in a different VLAN, each network engine will be configured as a BBMD by the ADX/ADS . This will result in BACnet broadcasts in any VLAN to be propagated to the other VLANs, in essence combining the smaller VLANs into one large VLAN from a BACnet broadcast perspective. Broadcasts for other protocols (e.g. ARP, DCHP, etc.) will still be limited to the smaller VLANs.
Placing all the network engine s together in their own separate VLAN, as proposed in Segmented Metasys BACnet/IP network, can help limit the scope of BACnet broadcasts. However it would require IT to configure directed broadcasts between the VLAN containing the supervising engine and the VLANs containing the controllers, something IT may be unwilling to do. VLAN C is a general purpose VLAN in the IT network, containing an IP controller configured as a BBMD (a BBMD is needed in VLAN C in order to receive discovery messages and other BACnet broadcasts from other subnets/VLANs).