Detected Circular Networks

Overview

Circular networks occur when more than one path is created from a device to a BACnet network.

When a device sends out a BACnet broadcast packet (e.g. Who-Is) anywhere in the system, that packet will be routed by both BACnet routers to the a single BACnet network (e.g. a BACnet/IP network), which then gets routed again by the other BACnet router, and round and round it goes.

In a single MS/TP network, the situation as illustrated below (Figure 1)  with two routers to a BACnet/IP network is rare, but can occur.

More commonly, routes for packets are sent are across both a BACnet/IP and a BACnet/Ethernet network to one destination, communicating on both networks simultaneously (as seen in Fig. 2). When a device sends out a BACnet broadcast packet (e.g. Who-Is) anywhere in the system, the packet will be routed by both BACnet routers from one network to the other networks (e.g. BACnet/IP → BACnet/Ethernet), which then gets routed again by the other BACnet router back onto the original network, and round and round again.

How The Diagnostic Works

To identify circular networks, OptigoVN looks for packets by source where the BACnet NPDU Hop Count have a large range of different values. This would indicate that the same packets is going around and around the network, with the Hop Count counter decreasing each loop.

How To Fix It

Circular Networks typically trigger a short-lived broadcast traffic storm, so you won’t be able to reach any of the controllers.

The BACnet routers that are routing these packets in a circular path can be identified from these packets. Highlighting the BACnet routers is valuable information for our users. You'll need to unplug one of the problematic controllers, isolate it from the network, and reconfigure it. You can then add it back onto the network.