Troubleshooting a BACnet network is like being a detective. To quote the great TV detective Columbo, “There’s a reasonable explanation for everything if you just put your mind to it.” So, when things start acting “weird” on your BACnet system, there’s almost always a “reasonable” explanation for what’s happening. In our experience, duplicate BACnet addressing is one of the first reasonable things to look for. 

Duplicate BACnet addressing occurs when two or more devices are unintentionally assigned the same designation – appearing to the rest of the network as one device. Duplication issues can be tricky to diagnose on their own: they can occur across many different hardware scenarios, something as simple as a lack of coordination between vendors, and tend not to appear as an obvious symptom in manual troubleshooting. Fortunately, OptigoVN makes finding duplication a breeze. 

Let’s look at three common forms of duplication – duplicate device addressing, duplicate BACnet instance numbers, and duplicate network numbers – and how you can use OptigoVN Site Scopes to resolve them in minutes instead of days. 

What’s the Problem with Duplicate BACnet Addressing?

Optigo Network’s Co-Founder and CTO Pook-Ping Yao has an analogy he uses to describe duplicates. “Imagine a scenario where more than one person has the same name. ‘Hey Bob, what do you want to eat?’ If there are two Bob’s, which answer do you use? Duplication in BACnet works the same way. The “client” (sender) may get two answers, not knowing which is the “right” answer, the client may take both answers to decide what to do.”

Duplicates can cause a range of issues across your BACnet system that can manifest in strange ways with seemingly no pattern: 

• Communication failures: duplicates can cause conflicts, including data flowing to the wrong destinations or along the wrong routes, or devices disappearing then reappearing on your network. This leads to lost packets and data collisions.
• Device isolation: duplicate devices can become isolated from the rest of the network, or experience communication failure.
• Data integrity issues: inconsistencies and conflicting data results occur when duplicates try to report back to the same read request. 
• Control malfunctions: automation and control programming will be disrupted by trying to communicate with two conflicting devices.

1. Duplicate Device Addresses

Duplicate device addresses are a fairly straightforward issue that can be tricky to track down. Every device on any network needs to have a unique source address they can share to let other devices know how to communicate with them. It’s worth noting that different MS/TP network segments can repeat device (MAC) addresses, but never within the same segment. Whether it’s a MAC address on an MS/TP network or an IP address on a BACnet/IP subnet, the addresses must be unique for the network to function properly. 

Think of it like a house number: while different streets can have the same house number, every house on the same street has to be unique if you want anyone to be able to find you.

How to Solve Duplicate Device Addresses

Since assigning device addresses can sometimes be a manual process in BACnet, the possibility of creating duplicate device addresses is high, especially in systems involving multiple vendors and contractors. 

If you’re using OptigoVN, you’ve got an ace in the hole. OptigoVN’s diagnostic suite has a specific test for duplicate addresses, and the additional information from a Site Scope will provide the BACnet instance numbers of the problem devices. From there, it’s a simple matter of reassigning those devices unique addresses to resolve the issue (make sure to re-run your diagnostics to confirm!).

There are also a few best practices you can incorporate into your management routines to help prevent duplicate addresses from getting assigned:

• Keep an up-to-date list of device addresses available in a centralized location, available for everyone to reference when commissioning new devices. Don’t forget to update it any time there’s a change! OptigoVN can help by providing a list of all BACnet device addresses in your captures.
• Periodically review device configs to ensure the address you have documented is still assigned.
• Make the practice of avoiding duplicate BACnet addressing through documentation and unique addressing part of your onboarding.

2. Duplicate Device Instance Numbers

Also known as a BACnet Instance Number and BACnet Device ID, it’s a unique identifier for each device on a BACnet system. If devices A and B are aware of each others’ instance numbers, they can communicate from anywhere on the network, even if moved. 

Unlike device addresses, BACnet instance numbers are globally unique. No two devices anywhere on any part of your BACnet system can have a repeated instance number. 

Back to our analogy of a home. Pairing your home number, say “123”, with your street name “Fake Street” makes your address even more unique. While there can be several homes on other streets with the number 123, and many other home numbers on Fake Street, there can only be one “123 Fake Street” anywhere in your area. 

A common duplicate BACnet addressing issue that leads to duplicate BACnet instance numbers is human error. Because just like device addresses, BACnet instance numbers need to be manually assigned. New hardware will typically arrive with a default BACnet instance number, and it’s all too easy to forget to change it. Another common mistake is to just copy and paste the configuration from another device, carrying over that Device Instance Number. Soon you’re seeing network issues like the duplicate address issue: communication errors, devices appearing and disappearing, and inconsistent data reporting.

How to Solve Duplicate BACnet Instance Numbers

Resolving duplicate BACnet instance numbers is essentially the same process as resolving duplicate addresses. OptigoVN’s diagnostic reports will alert you to any duplicate BACnet instance number issues, and a Site Scope pinpoints the duplicate devices’ unique addresses. From there, you can reassign unique BACnet instance numbers and re-test to see if the issue is resolved.

Preventing duplicate BACnet instance numbers also follows the same best practices. Inventory management will help you keep up-to-date records for reference, and periodic reviews of device configs can provide confirmation your documentation is correct. Integrating these housekeeping duties into your training should help significantly cut down on duplicate assignments.

3. Duplicate Network Numbers

This error has to do with BACnet routers, specifically, a situation where two or more BACnet routers are claiming to be the correct router for the same network. Think of them like street names – If you have more than one street with the same name, you can’t properly identify where the package came from, and worse, you aren’t sure which street your package will be delivered to. 

And just like floors in an elevator bank, every BACnet router needs a unique network number so data exchange between devices can happen seamlessly.  When two or more BACnet routers have duplicate network numbers, it can lead to communication conflicts and disrupt normal operation.

Worth noting: BACnet devices with routing functions often have their network ports enabled by default, which also carries a default Network Number. Leaving default ports enabled with default network numbers are often the cause of these conflicts.

How to Solve Duplicate Network Numbers

Fixing duplicate network numbers with OptigoVN is fast and easy. The diagnostic report will alert you to any duplicate network number issues, and with your Site Scope insights, pinpoint the unique device addresses of the two routers with overlapping network numbers. From there, you can reassign unique network numbers, and re-test to ensure the issue is resolved.

Good preventative best practices are once again your go-to for preventing duplicate network numbers: 

• Disable unused network ports to avoid overlap of network numbers for unused networks.
• Maintain an up-to-date inventory of BACnet routers and their assigned network numbers.
• Document device assignments and keep up to date with changes.
• Ensure team members involved in device configuration and management are aware of the importance of unique network numbers.
• Monitor network traffic and communication to detect signs of conflicts caused by duplicate network numbers.

Duplicate BACnet addressing issues can wreak havoc on your OT networks, but there are easy solutions. Along with a solid preventative maintenance program, OptigoVN’s best-in-class diagnostic tools will give you deep insights and fast resolutions to these common problems. Ready to see what OptigoVN can do for your network? Learn more and sign up for a free account now.