investigating network loops

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Troubleshooting the most common “things are slow” complaint is one of the most difficult calls I get since there are many scenarios and variables to consider.
Root causes range from the malicious (like DDOS or viruses) to the unintentional (like equipment misconfigurations), and everything in between.
I found the most effective approach is trying to eliminate the possible root causes that are easy to prove or disprove. For example, starting up your protocol analyzer of choice connected to any port on the client VLAN could identify layer 2 loops, devices sending out a lot of broadcast multicast packets and misconfigured load balancing.
In this example, when I got the call and performed this check, I got lucky and there it was a layer 2 loop. Many analysts skip this check because they rely on Spanning Tree or Rapid Spanning Tree to prevent this from happening.
In the video below I walk you through how I figured this out and what the issue was.

Enjoy

[andyjarman]

Excellent your videos are like having a personal Network Analyst Tutor, please keep posting them.

[shiiqa]

Thank you for your help. What is the software name of the analyser you are using.

[dollarsignsauce]

When the Wifi is bridging back into itself how do you stop it from Looping? Do you turn it off or stop it from looping?

[alittax]

Thank you, this was interesting! Without a network sniffer such as WireShark, is it possible to determine if there's a loop? Maybe if the interfaces and the CPU shows an unusually high utilization for some time period, or by looking at the "Received... broadcasts" interface output, or the input/output rates, or something else? I'm mainly considering Cisco devices. Thanks.

[mikecullen]

Great video 👍only just getting into wireshark as a novice, Is there a way of finding if 2 devices on the local are using the same IP address using WiFi? Thanks again and I'm definitely subscribing 😎

[petpeeve]

Whats the same if that application? Dope video by the way man very easy to do

[Camped_on_Dartmoor]

We had an incident at work where all the computers crashed all at the same time. A loop back was discovered on a un managed switch.
How long does it take for this to impact a network ?

[bengrogan]

This video cuts nicely to the quick for explaining how to detect if a loop exists - but misses a key part of the final understanding

Once you had the repeated packet, how did you figure out the physical interfaces that where causing the looped traffic?