What Does "Loop" Mean in Networking — And Do You Have One in Your Network?
Today
I headed to a company that had called me about a problem affecting all of their machines — roughly 40 computers. Over the phone, the issue was described as extremely slow devices that couldn't open a web page or even access a shared file.
I arrived on the scene puzzled and wondering what could be slowing down every single machine. My first thought was that a virus had spread from one device and hit them all. In the first minute on the machine I couldn't move anything — the mouse was sluggish and the computer would respond after what felt like an eternity; even Task Manager refused to open. Damn virus 
I rebooted the machine (of course I had to do a full shutdown), then booted into Safe Mode — the computer ran normally. I tried scanning it with a few antivirus tools but found nothing (a conspiracy!). My second decision was to isolate the machine and disconnect everything connected to it — USB devices, the network cable — except the mouse, which I left for testing. The problem went away. I reconnected everything as it was before and the problem came back immediately. At that point I concluded that there was a fault on the network, and that some device was flooding the entire network. I went to the switch and found a standard 48-port switch. I unplugged all the cables and split them into two halves — twenty and twenty. I connected the first group and the network worked fine and all machines were OK. I then split the second twenty into two groups of ten. I connected the first ten and the problem came back, so I knew one of those cables was the culprit.
Using the same method and approach, I split the ten into two groups and kept testing until I tracked down the cable causing the problem. I traced the cable and found it was connected back to the switch itself — meaning we had a classic loop situation in every sense of the word. It was the first time I had ever seen a real loop inside a live network.
Also:
In networking, the term "Loop" refers to the existence of a repeated connection or closed circuit between two or more devices on the network. This typically occurs when two switches are connected to each other by more than one cable, or when a single cable is plugged into two ports on the same switch.
When a loop occurs, data traffic between the devices on the network is duplicated endlessly — a packet is sent from the first device to the second, then to the third, and then back to the first, and so on. This leads to a massive increase in data circulation and monopolization of network resources, causing the network to slow down and malfunction.
To address the loop problem, loop-prevention protocols are used, such as Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP), as well as Loopback matrices in managed switches. These protocols detect loops and disable redundant links to prevent loops from occurring and ensure the network operates correctly and efficiently.
Furthermore:
If there is a Loop in the network, what is known as a "Broadcast Storm" may occur. This involves a large volume of frames spreading across the network through all ports, causing a noticeable spike in data traffic that can lead to a complete network outage and disable all connected devices.
A broadcast storm causes a significant increase in the amount of data that devices must process, placing enormous pressure on the CPU and its ability to analyze and handle the massive volume of data. You may notice the "Received" counter on the network card climbing rapidly until it hits its maximum value and stops counting.
To protect yourself from this problem, you can take the following measures:
- Avoid connecting the same cable to the same switch. Make sure each cable is connected to a different port on the switch to prevent a loop from forming.
- If you have cheap or non-advanced switches, avoid connecting more than one cable between those switches. Only one cable should connect each pair of switches.
- Most switches offer a feature called "Spanning Tree Protocol" (STP), which helps prevent loops from forming in the network. You can enable this protocol on your switches to avoid a Loop.
- On some Cisco devices, there is a feature called "Storm Control" that helps manage the Flood problem inside the network. You can look up additional information about this feature to enhance your network's protection.
We hope you found this information useful and wish you success in protecting your network.
And with that, my friend, we have successfully completed the mission 
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