My internet speed is supposed to be 100Mbps down and 10Mbps up, if I connect my PC directly to modem I can measure that, with a degree of error. So far so good. But if I connect PC to modem via router then my speed drops to 10Mbps/10Mbps. I tried Asus RT-AC51U and TP-LINK Archer C20 and both routers drop connection to roughly the same speed even though they should support speeds up to 100mbps. I don't know how to troubleshoot any further, any help would be much appreciated.
EDIT: I used same cable for PC-modem connection as for PC-router connection. I always used same port on modem but tried all 4 ports on router. I can further try other 3 ports on modem and change cable for model-router connection. Will report on that.
41 Answer
Auto-negotiation of the speed between devices doesn't always result in a correct connection.
There is a reason it is called auto-negotiation of connection speed and not auto-detect. The negotiation can fail and in that case both devices are supposed to fall-back to a default.
Unfortunately there is no good definition in the auto-negotiation specification what that default should be. So each device can do whatever it wants. Some go to their highest supported speed, others to their lowest.
If both ends of the connection make a different choice this leads to a mismatch resulting in slow speeds.
This could lead to the router thinking it is connecting to a 10/10 modem instead of making a 100/100 connection.
In the settings of the router check the WAN port settings and see if you can force the connection to 100 Mb/s, full-duplex. That usually solves this problem.
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