Configuring directed broadcasts - Metasys - LIT-12012458 - Field Device - 13.0

Metasys IP Networks for BACnet/IP Controllers Technical Bulletin

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As noted in Configuring virtual local area networks (VLANs), VLANs are used to isolate a broadcast domain in an IP network from the rest of the IP network, such that under normal circumstances IP broadcasts will not be sent between VLANs. If BACnet broadcasts are required between VLANs, BACnet Broadcast Management Devices (BBMDs) can be configured on the network engine s in each of the VLANs. Alternatively, such as in the case of a Segmented Metasys BACnet/IP network where the network engine is not in the VLAN of the devices which it is supervising, IP helper addresses and directed broadcasts can be configured to forward IP broadcasts between specific VLANs.

The following steps are required to forward IP broadcasts between two VLANs on a Cisco managed switch or between VLANs on different Cisco managed switches. In this example, IP broadcasts are forwarded from VLAN 5 (which hosts the /29 subnetwork) to VLAN 10 (which hosts the /24 subnetwork). A complementary configuration would be required to forward IP broadcasts from VLAN 10 to VLAN 5.

The following steps are required to configure directed broadcasts from VLAN 5 to VLAN 10 on a Cisco managed switch. These steps assume that the SVIs for VLAN 5 and VLAN 10 have already been configured and that IP routing has been enabled as per Configuring IP routing and IP routes.

Table 1. Configuring Directed Broadcasts from VLAN 5 to VLAN 10 on a Cisco Managed Switch

Configuration Step

Cisco IOS CLI Command


Enter global configuration mode.

Switch# configure terminal


Enable forwarding of BACnet traffic. In this example, it is assumed that BACnet is using the default port number of 47808

Switch(config)# ip forward-protocol udp 47808


Enter configuration mode for the SVI for VLAN 5.

Switch(config)# interface VLAN 5


Add an IP helper address specifying the subnetwork-specific broadcast address of the sub network to which IP broadcasts originating in VLAN 5 should be propagated.

Switch(config-if)# ip helper-address


Exit configuration mode for the SVI
Note: In this example it is assumed that VLAN 5 and 10 are on the same switch such that we can remain in global configuration mode. In practice, VLAN 5 and 10 could be configured on different physical switches in which case steps 1-4 would be run on the switch configured with VLAN 5 and steps 6-8 would be run on the switch configured with VLAN 10

Switch(config-if)# exit


Enter configuration mode for the SVI for VLAN 10

Switch(config)# interface VLAN 10


Use the subnetwork-specific broadcast address ( instead of the default of as the IP controllers do not respond to

Switch(config-if)# ip broadcast-address


Exit global configuration mode.

Switch(config-if)# end

Note: ACLs can be set up at the receiving SVI to accept directed broadcasts only from specific devices (for example, from network engine specifics). For additional details, see Applying ACLs to an interface and Configuring directed broadcast IP ACL.