Network loops are one of the few pieces of evidence that the universe loves to laugh at us. Somewhere, right this minute, a five-dollar Ethernet cable is taking down a million-dollar infrastructure. An innocent intern trying to get internet in the conference room has both ends of a patch cable plugged into the same switch, and now 500 employees are staring at the spinning wheel of doom. Meanwhile, the CEO's slideshow presentation is stuck on the same slide for the umpteenth time, and the accounting department rocking back and forth crying, "Remember the good old days of the pre-internet world?"
And you are about to spend the next two hours crawling under desks, reaching into cable ducts, and otherwise channeling your inner Indiana Jones on a quest to find The Cursed Ethernet Cable that's bringing down your digital empire.
Network loops are one of those problems that you know how to fix in theory but are maddeningly hard to pinpoint in practice, especially in a busy data center or with a complex topology involving multiple switches, VLANs, and redundant paths. But knowing why they happen and how to spot them early can help you avoid some truly miserable days.
A network loop occurs when you unintentionally create a circular path between switches. Network traffic enters the loop and just keeps circulating, and there is nothing inherently built-in to stop it. Every time the traffic goes around the loop it gets duplicated and forwarded again, resulting in an exponential traffic storm that floods your switches in seconds.
Typically, the root causes are accidental human error and misconfigurations. The most common scenario is someone (maybe that innocent intern) plugging in an extra Ethernet cable with good intentions, to provide a redundant link, extend coverage, or just to "make sure it works." In doing so, they actually create redundant links that form a loop. This often happens by connecting two ports on the same switch or creating a circular path between multiple switches. Unmanaged switches are the worst because they have no loop detection. Office moves, temporary setups, conference room quick-fixes are all common situations that can lead to these mistakes and trigger network outages.
Spanning Tree Protocol (STP) was created to stop loops by automatically detecting redundant paths and blocking them, resulting in a loop-free network topology. However, STP isn't always reliable, can be disabled accidentally on network devices, or simply be overwhelmed by frequent topology changes. If it fails or isn't enabled, one mis-placed cable can cause a broadcast storm that will flood your switches, routers, and even firewalls with duplicate frames, bring your entire network to its knees, and cause a network outage.
The trick to solving network loops is early detection through effective loop detection. Preventing the problem from ever occurring is great, but prevention is only good if you can't see a loop once it starts. And network loops aren't exactly the most visible of network issues; you often don't even know there's a problem until 15 minutes of frantic cable plugging have been tried.
First, you need visibility into your network traffic - and that's where network monitoring tools like PRTG Network Monitor come in. Let’s take a look at some PRTG network loop monitoring strategies.
The SNMP Traffic sensor is your first line of defense in network loops. You can immediately spot the early signs of a potential loop by monitoring the bandwidth utilization of all your switch ports, looking for abnormal spikes in broadcast traffic and multicast packets that are the precursor to a broadcast storm. If you see one or more ports on your switches jumping from a few megabits to absolutely maxing out the interface utilization within seconds, that's a red flag. On a healthy, normal network, you'll see a good mix of unicast, multicast, and broadcast traffic. When you have a loop, the broadcast and multicast ratios are sky high. Set threshold alerts on this sensor so you know something's wrong before your users start calling the help desk.
Network loops generate a ton of traffic that switches have to process, and this quickly pegs the CPU at 100% utilization. PRTG's SNMP CPU monitoring sensors can monitor the resource usage of your switches and alert you when a switch crosses the critical threshold. If you have an access switch that normally doesn't see a lot of use, and suddenly it starts running at full utilization, you know something's up and it's time to find out what.
Broadcast storms can also cause network devices to drop off the network as switches, routers, and other network infrastructure get overwhelmed. PRTG's good old Ping sensor can track the availability of network devices across your network. If you see large numbers of network devices suddenly start turning red in your maps and device tree, that's a pretty good indicator that something is seriously wrong with your network.
Prevention is always better than cure. But what are the best practices for actually preventing network loops in the first place?
Enable and correctly configure STP on all your network switches to ensure that they prevent network loops. Use Rapid Spanning Tree Protocol (RSTP) or Multiple Spanning Tree Protocol (MSTP) on modern switches for better performance and faster convergence. Ensure the root bridge is the correct location, not chosen randomly based on MAC address, and document your STP topology (Maps in PRTG can help here) and review it regularly when you make network changes - this goes for all managed network switches, including Cisco and Unifi, as well as smaller platforms like Ubiquiti, MikroTik, Meraki, etc.
Use port security on your switches, if available, to help prevent loops. Configuring BPDU Guard on access ports can help prevent someone from plugging in a switch where it shouldn't be. Loop Guard can help detect and disable ports that are at risk of forming a loop, and Storm Control can limit the amount of broadcast traffic allowed on a port, helping to contain a loop if it does occur. These built-in loop prevention mechanisms can help provide a more robust and loop-free network topology.
Use different colored Ethernet cables for different purposes. Use one color for uplink connections, one for access ports, another for out-of-band management. It's not rocket science, but this simple visual differentiation helps a lot of avoid misconfigurations, especially during the late-night maintenance sessions when the network administrator is fueled by coffee and adrenaline - and it might just make that intern think twice before messing with the cables.
Use PRTG to establish a baseline for normal traffic patterns in your network. When you have a sense of what's normal, abnormal becomes immediately obvious. So, set up the custom sensors you need to monitor the specific metrics that matter for your environment, then configure notifications that will actually get your attention when something goes awry.
A mid-sized company was setting up for a board meeting, and an AV technician brought in a small network switch to connect the presentation equipment - and for "redundancy" plugged both of its uplink cables into the corporate network.
Within seconds, the entire floor's network went down.
Their PRTG installation immediately reported bandwidth utilization jumping on multiple switch ports and CPU utilization maxing out on three access switches. The network administrator got an alert on his phone, accessed PRTG remotely on his tablet, and could immediately see which switch ports were reporting abnormally high broadcast traffic. He called the AV technician, had them unplug the offending loop cable, and in less than five minutes was done. A quick check in Wireshark (never leave home without it) confirmed the broadcast storm had stopped, but without PRTG's real-time network monitoring, they would have spent an hour manually tracing cables with no idea where to start. The board meeting started on time.
During an office renovation, a company's contractors moved a number of network drops and reconnected them to a temporary switch to keep employees working during the office refurbishment. However, one contractor accidentally created a loop by connecting two ports on the same switch to two wall jacks which were already connected to each other through the structured cabling.
PRTG's traffic monitoring immediately showed a spike in broadcast traffic on the VLAN affected by the loop, and the IT team received alerts even before users noticed any problem. They used PRTG's maps to immediately see which core switch was affected, checked the physical location, and found the redundant links. Proper troubleshooting and loop detection had eliminated the problem in minutes instead of causing hours of network outages.
Network loops are one of those problems that almost feel embarrassing when you discover what's happened. All that chaos, all those frantic users, all because someone accidentally plugged a cable in the wrong port. The thing is, they're also completely preventable with the right monitoring and the right measures in place.
PRTG Network Monitor can give you the visibility you need to spot network loops before they become catastrophic. With bandwidth monitoring, device availability tracking, and switch performance monitoring, you'll know something's wrong seconds after a loop has started. Combined with proper STP configuration and good network topology documentation, you can build a network that's resilient to accidental loops.
The best part about preventing network loops? You never have to look your boss in the eye and explain how a five-dollar Ethernet cable just cost the company thousands of dollars in lost productivity. Nobody wants that conversation.
Want to protect your network from loops and other infrastructure disasters? Download a free trial of PRTG Network Monitor and get 30 days to try out all the sensors and features. No credit card required, no sales calls, just full-featured network monitoring that actually works (please note PRTG cannot monitor interns with cable swapping tendancies - but prettty much everything else is covered).