New Internet service from Spectrum, but ZeroTier slows it down, very odd behaviour. Why?

Greetings. Recently we upgraded to a new faster internet service from Spectrum. Service uses Coax cabling and comes with a modem device and a separate router from Spectrum. Being a business we requested a static IP address. We already have our own router so Spectrums router is set in to bridge mode.

We configured our own router, a Fortinet FortiWifi 60F with the static IP address, SM and GW they gave us. All is configured properly and is working properly. Speeds are what we are paying for 600 MB down and 40 MB up.

But a soon as one of Windows workstations on our network starts the ZeroTier service, our Internet connection slows down. Pinging our router static IP address remotely, we get “Request timed out” followed by a successfully Reply, followed by a “Request timed out”. Funny that at times, for about 1 minute, we get continuous successful pings and during this time we can remotely ping the ZeroTier IP address of the workstation. But when it is slow, we can’t connect via ZertoTier even though the dashboard says the client is online.

Computers on our network, do the same when pinging Google DNS and download speeds drop in half from 600 MB to 300.

After 10 seconds the ZeroTier service is stopped on the Windows Workstation, speeds go back to normal and no more dropped pings from the outside and the inside.

I have followed the steps and recommendations on this ZeroTier Router Configuration tips page:

But still does not work. I have check peers and they are connected as direct. I have checked and we are not double NATted.

What else can I check? Thanks for your help.

There’s nothing inherent to ZeroTier that would cause this. Especially as, as you report, it affects the internet speed of things ZeroTier is not running on. Seems like an issue with your router and/or internet provider, and that’s not something we have any insight into.

Thanks for your reply. On reddit someone suggested to check if traffic shaping was enabled on our FortiGate. Traffic shaping was enabled but I was not using it. I have disabled it, but it did not help. As soon as only one workstation on the local network starts the ZeroTier service, the problem of slowing down the network begins.

In my desperation I installed Tailscale and have NO problems. No dropped ping requests or a drop of download speed. I know ZeroTier and Tailscale use different protocols for connection, but I wanted to see if a similar product acted the same and it does not.

Again ZeroTier keeps dropping ping requests and slows down the Internet as soon as I start the ZeroTier service on one computer on the network.

Yesterday after stopping the ZeroTier service, the problems did not go away after 10 seconds like I noticed before. I had to restart our Fortigate router to fix.

Could I be dealing with an issue with our ISP, Spectrum? Maybe they are limiting ZeroTier traffic? This reddit post has more info about Spectrum:

It’s definitely an issue with your router and/or ISP. Beyond that I cannot say.

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