What It Is
Who-Is is a BACnet discovery service used to locate devices on a network. A client sends a Who-Is request, and matching devices respond with I-Am.
It is one of the reasons BACnet is easier to inventory than protocols such as Modbus, which generally rely on vendor documentation and point maps rather than native discovery.
Request Types
| Type | Behavior |
|---|---|
| Global Who-Is | All BACnet devices in scope may respond |
| Bounded Who-Is | Only devices within the specified Device Instance range should respond |
Where Discovery Fails
Who-Is problems are rarely about the message existing at all. They are usually about path conditions:
- wrong adapter or network path,
- blocked UDP broadcast traffic,
- BBMD or FDR gaps across subnets,
- duplicate Device Instance values,
- client expectations that differ from the actual response behavior.