VPN connection crashes when downloading large files over 20 MB

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Level 1

4 points

VPN connection crashes when downloading large files over 20 MB

Hello Everyone,

so i am working as a freelancer and am connected via VPN to a server of a client. I had a 100mbits connection and decided to upgrade it in order to just be able to work faster. So I went to my IP and upgraded to a 1000mbits connection. That's when my problems started. When I was connecting to the VPN there is no problem at first I can browse the whole server with speeds that made my mouth watery. But as soon as I click on or download a file that ist over 20 mb in file size my connection crashes. First oder of business was to contact the ADMIN of the Client asking him, if he can see something and check if there is something wrong with my account or something. We redid everything, configured this and that no change. He was like "sorry but since it is working on my end and with others the problem must be on your end". For me taht made sense aswell. So I tried connecting via hotspot on my mobile phone. And what you know … it works. Mh so I thought .… okay that must be an issue with the IP or something. Meanwhile my girlfriend could use her connection to another VPN Server perfectly.

So i talked to the IP describing the problem – they send someone over checked my connection downgraded it to various speeds no difference. Talking to them in the next weeks and they were doing something with DS Lite and Tunneling I could not really understand but they said "they can sometimes see the problem but don't know really what's wrong" … then they wanted to exchange the router I got … 4 weeks of waiting for the new fritzbox made no changes.

Meanwhile … I still need to work so I am reading and testing and what not. I changed the router settings to max stability / lowest speed and split the connection into 2,5ghz and 5 ghz. Then I played around with the MTU a bit but no real difference. One Day my girlfriend was working alongside me and I did a speedtest … my speed was under 80 mbits for the 2,5ghz I connected to the VPN and it worked … slow but it worked. I thought mhhhh feels like the speed seem to be an issue. I reconnected the 5 ghzs did a speedtest … went up to 800 mbits … vpn instantly breaks again when I just click a bigger file on the Server. I thought that is crazy. I connected my mobile hotspot again but positioned it in a good spot so I have a good connection … it also breaks. So I thought okay maybe something with the speed but as long as it work with the 80 mbits its okay for this client. When using anything else I can go for the fast connection.

Next day I wanted to do some work … connected to the VPN … breaks again. The speedtest for the 2,5 ghz said over 120 mbits … mhhhh. I tried to install a downgrading app for my bandwidth but this prevents me from connecting to the VPN completely. I started to read again online what the issue could be I have tried every way of connecting to the inter / router … cable / 2,5 ghz / 5 ghz/ combined … no change. I even asked chatgpt to help me … the answers were all over the place and most of them were aimed at changing stuff on the server or using wireguard (which I cannot because the admin won't use another protocol because it still works – understandable).

So just yesterday I was tinkering again playing around with the MTU in my settings (for the 50th time) when I put in 1460 for my 2,5 ghz connection I tested it and … what do you know it works. I was like oh that's it ? I went to my faster connection (5ghz) put in the same numbers connected to the VPN … disappointment. I went for the ethernet connction put in 1460 MTU swell … disappointment.

Since then I wrote a script with chatgpt to check for the perfect MTU settings to check with 8.8.8.8 for google – It will give me a value that works but when pinging the VPN server with the same script directly it won't give me a working MTU. Using the MTU Value of the google ping results in a somewhat working connection with the VPN as well BUT only when using the slow internet as described before … and it still crashes when loading bigger files then 300 mb +.

So I guess the issue is this …? Can somebody help me?

Regards!

[Re-Titled by Moderator]

Original Title: VPN Connection issues

Mac mini, macOS 15.6

Posted on Aug 18, 2025 02:05 AM

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Answer 1

Top-ranking reply

Camelot

Level 9

57,885 points

Aug 19, 2025 12:31 PM in response to toaiwehao

This certainly sounds like an MTU issue.

> Since then I wrote a script with chatgpt to check for the perfect MTU settings to check with 8.8.8.8 for google

Checking against 8.8.8.8 is pointless. MTU only becomes an issue on large transfers - it defines the largest single packet size, and larger transfers are chunked into packets of this size for transmission..

8.8.8.8 is Google's DNS service. You're not likely to generate any DNS query/response that comes close to overwhelming the MTU and therefore triggering the problem.

The real problem is that MTU is subject to the lowest common denominator anywhere in the chain between source and destination. For the most part, everyone works on the same standard/default, which is why it's not normally a problem, but things like VPN add overhead for the encryption protocols. Usually the VPN software will take care of adjusting the MTU when the connection is established, but this may only work from your device up to the router. If you're doing funky things like dual-NAT (e.g. you're talking to one router that does NAT, which then talks to another router that also does NAT, that second NAT device might not get the MTU message, and hence causes the problem.

The problem can also be compounded by IPv6 vs v4, wired vs. wireless connections, and a whole slew of other things.

I'd start by simplifying the network as much as possible - are you connected directly to your ISP's router? or do you have some other device in the way? Get rid of that intermediate device if you can (at least temporarily) to see if that's the problem.

Given the 2.4GHz vs 5GHz discussion, it sounds like you're connecting wirelessly? Do you have an option to use a wired connection (again, temporarily)?

Try turning off IPv6 and see it that helps.

Try lowering the MTU even more.. .go stupidly low (like, 900) for testing and increase until the connection breaks.

Finally, for now...

> Meanwhile my girlfriend could use her connection to another VPN Server perfectly

Do you know what VPN they're using? vs. your company?

If possible, can you connect your machine to her VPN successfully? what about using her machine to connect to your VPN? That would help eliminate/narrow down where the problem is, if it's specific to one machine.

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Answer 2

toaiwehao Author

Level 1

4 points

Aug 20, 2025 02:43 AM in response to Camelot

Hello Camelot,

Thank you for your Reply! Okay checking against google is pointless – I thought about this aswell :).

I am directly connected to a router (fritzbox) no devices in-between. I cannot disable IPv6 since its a IPv4 over DS-Lite – if I click on native Ip4 I don't have an internet connection at all.

Yes I use wifi – since when I am using a wired connection there was no scenario in which I could get a stable connection to the server. Which for me is not understandable, but that what I was trying to say: when I connect with the faster connections methods its crashes in every case, not just big files.

I cannot lower my MTU below 1280 – I guess it is because of the IPv6 Connection – google says ipv6 is limited to at least 1280.

I cannot use the VPN of her company and she cannot use mine since its connected to accounts which are been set from the server admins.

They are using Wiredguard but the admin of my company said we do not implement this on our system because only I have issues. Myself I use the native VPN software of apple.

Regards Tobias!

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