Switch - How to know if a loop is ongoing in your network?

In support we are quite often contacted with symptoms like:

• Slow network
• Can not browse the internet / web pages load very slow
• Downloads or file transfers are slow 
• I have to restart my switch any 1 or 2 days to get my network running again, but it gets gradually slower again

Of course these symptoms can have different root causes, but please be aware that in not only a few cases this was caused by a network loop or the broadcast storm coming with it.

Here are some indicators which can potentially be pointing to a loop:

• "Port flapping": Ports of switches going up and down high-frequent without an explanation or a person on-site disconnecting devices
• Links between devices and a switch are establishing with an unexpected rate (a computer that normally should have 1000M only establishes with 10M or 100M) (often in cobination with port flapping)
• In Nebula you might even explicitely see the message "Broadcast Storm detected".

There is also of course the way to get a hard proof by using Wireshark and a packet capture to check if one and the same frame is appearing with a high frequency on the port you are capturing on.

Examples of potential sources for loops:

In the above graphic you see a redundant link between two switches.

Redundant links are often needed in a network to ensure availability. But you will need a managed switch where you can configure Link Aggregation on!

The graphic to the right shows three switches, which are connected in a loop. Sometimes this can happen by accident when connecting cables without knowing the whole network topology. But sometimes sucha  topology is actually wanted, once again to ensure high availability if a switch or port of a switch dies. In that case you will need Spanning Tree.