I am seeing a strange upload issue with a DrayTek Vigor3912S. Test setup:Fiber ONT → Vigor3912S → PC directly connected to the Vigor3912S 2.5GbE portThe PC is connected directly to the router with a high-quality cable, so cable runs, wall outlets and switches are excluded.With the link running at 2.5GbE, I get approximately:Download: 920 Mbit/sUpload: 200 Mbit/s If I force the same connection to 1GbE, I get approximately:Download: 920 Mbit/sUpload: 920 Mbit/sSo the problem only appears when using the 2.5GbE link mode. At 1GbE, upload is normal.Are there any known issues with the Vigor3912S 2.5GbE ports causing poor WAN upload performance?Which settings should I check?I am using the latest Firmware versionAny advice would be appreciated.

What happens when you force it to 2.5gb rather than auto?
 

That does not solve the problem.

Interesting
Do you have any other equipment which is 2.5g to test with ?
You can also try another cable.

Finally if your connected is 1gb both ways you may as well use it at 1gig.

Yes, I tried several devices. Always the same behaviour. I don´t have the option to use it at 1Gbit because I have another 1Gbit WAN and need the full speed.

Would be interesting to put another switch between 3912 and the ont too see what it does then.

I'm sure this has come up before perhaps with a 3910 just nagging at the back of the mind.

The other option is the 10gig ports you're going to tell me you're running a 10gig network on those.