What Who-Is Is
Who-Is is a BACnet broadcast discovery service used to locate devices on a network. A supervisor or client sends a Who-Is request, and every matching device responds with an I-Am.
Request Types
| Type | Behavior |
|---|---|
| Global Who-Is | No device instance range — all devices respond |
| Bounded Who-Is | Specifies a low/high Device Instance range — only devices within respond |
Multi-Subnet Considerations
Who-Is relies on broadcast. On a single BACnet/IP subnet, broadcasts reach all devices naturally. Across subnets, a BBMD on each subnet is required to forward Who-Is broadcasts and I-Am responses.
[!WARNING] Without BBMDs, Who-Is only discovers devices on the local subnet — even if IP routing between subnets is fully functional.
Troubleshooting No Response
- Verify the target device is powered and online
- Confirm Device Instance is within the Who-Is range (if bounded)
- Check that BBMDs are configured on all relevant subnets
- Verify no firewall is blocking UDP 47808
- Check for duplicate Device Instances on the network