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.

Another popular example are speaker systems. Some vendors or models do establish some kind of wireless mesh network between the speakers, but in addition the speakers do also have a network cable connection thus causing a loop in your network again. More info here: https://support.zyxel.eu/hc/en-us/articles/360011000819-Sonos-and-Spotify-Connect