Directed broadcasts - Metasys - LIT-12012458 - Field Device - Metasys BACnet/IP Controller - 13.0

Broadcasts are not propagated across VLANs. Directed broadcasts give the network designer fine control over the propagation of IP broadcasts across VLANs. This is particularly useful for the Segmented architecture.

The scope of an IP broadcast message is by default limited to the VLAN/subnetwork in which the broadcast originated. To propagate an IP broadcast across VLANs, directed broadcasts must be configured between the VLANs. If the network engine s are not in the same VLAN as the devices they are to supervise, directed broadcasts must be configured for the network engine s to discover the devices.

The following tables show the configuration required for the directed broadcasts on the BAS switches. Note that the local broadcast address (for example, 171.16.10.255) on the destination VLANs must be configured, as the IP controllers do not support the default broadcast address of 255.255.255.255.

Table 1 illustrates the directed broadcast configuration for the separate dedicated VLAN example shown in Figure 1.

Table 2 illustrates the directed broadcast configuration for the separate dedicated VLAN example shown in Figure 1. The configuration in Table 1 results in broadcasts initiated by any network engine in VLAN 5 to be propagated to all the BAS VLANs (VLANs 10, 11, and 12) in which the IP controllers reside. To limit the propagation of these BACnet broadcasts, an ACL can be added to each of the BAS VLANs which only allows the broadcasts from a specific network engine into the BAS VLAN in that the network engine 's IP controllers reside.