Global Who-Is

Overview

• Devices use Who-Is service (broadcast) to discover other BACnet devices on the network.
• Usually it’s targeted at a specific device, but in some cases they broadcast a Global Who-Is to the whole network, and they will all respond with I-Ams (broadcast).
• Global Who-Is should never be sent automatically, or should be used very sparingly so as not to flood the network.
• Fails if a single source sends more than one Global Who-Is during a capture.
• Any segmented Who-Is is included as a Global Who-Is.
• This is a way to break up the requests into many smaller requests that look for ranges with the entire BACnet device ID range.

Why it fails

• Misconfiguration of the BMS (constantly or regularly discovering the network).
• This could be a setting or incorrect default.
• Poor implementation of the discover process.
• Good intentions can cause this problem. Keeping an accurate BACnet device count is crucial to a network, especially with security, but the network can pay for that.
• Manual global discovery during the capture.
• This is a great way to get as much BACnet device information as possible for troubleshooting. Be careful, though, as it can have drastic effects on the network. As the network’s size increases, this becomes more problematic.
• General
• Many systems will do this check to make sure that the BACnet device list is up to date. There are other ways to do this, such as a targeted Who-Is, but every system will need to find the controllers on a network.

How to fix it

• Identify the source device causing the problems.
• Determine which device and how often it is sending it.
• Is there a regular frequency? Is it random?
• Is it the same source every time or multiple sources? It could be different servers or vendors.
• Check the configuration on the source device(s) sending the Global Who-Is.
• Is it normal? Can it be turned off or reduced?
• If manually triggered, note the time and the source so you can easily track it throughout the system.

Watch us troubleshoot Global Who-Is in this live webinar on a hospital site. Skip ahead to 18:22 for the Global Who-Is diagnostic check.