What I-Am-Router-To-Network Is
I-Am-Router-To-Network is a network-layer message in BACnet, defined in Clause 6.4.2 of ANSI/ASHRAE 135-2024. A router sends it to announce the network numbers it can reach. It is carried in an NPDU (Network Protocol Data Unit, the network-layer packet) rather than as an application service, so it is unacknowledged: it is always broadcast, and nothing replies to it. It is the answer to a Who-Is-Router-To-Network query, and a router also emits it on its own to advertise its routes.
Like the other network-layer messages, it operates below the object and property layer. It is part of how devices learn the path to remote networks, not something a client reads or writes.
How It Works
The message carries one or more 2-octet network numbers, each a network the sending router can reach. It is always transmitted with a broadcast address so every device on the local network hears it. A router sends it in response to a Who-Is-Router-To-Network naming a network it serves, and it may also broadcast it unsolicited, for example at startup or when its set of reachable networks changes, so other devices and routers can update their routing tables.
Where You’ll See It
- Route discovery. After a device sends Who-Is-Router-To-Network, the I-Am-Router-To-Network that comes back tells it which router to send remote traffic through.
- Router startup. A router announces its reachable networks so neighbors learn the routes it provides without having to ask.
- Topology diagnostics. Watching these announcements maps which router serves which network number, which is how a duplicate or missing route is spotted.
I-Am-Router-To-Network Error Codes
I-Am-Router-To-Network is an unacknowledged network-layer message, so it has no application-layer error response. Trouble shows up as a route that does not appear or a conflict, not as an error code. The usual causes:
- The router that should announce a network is offline or not configured for it, so no announcement arrives.
- Two routers announce the same network number, a misconfiguration that produces inconsistent routing.
- The broadcast did not reach a listener because of a subnet or BACnet BBMD path problem.
Profiles That Require It
The network-layer messages are the router’s job. Sending I-Am-Router-To-Network is required through the router configuration capability NM-RC-B, which is required of the router profile B-RTR. Non-routing devices consume the announcement but do not originate it.
What Chipkin Provides
The CAS BACnet Stack supports I-Am-Router-To-Network, emitting it both as the reply to a Who-Is-Router-To-Network and as an unsolicited route announcement. Talk to us about adding BACnet client or server support to your device.