I have an asus Gigabit Router and a gigabit NIC (Realtek PCIe GBE Family Controller) witch is running at 100Mbps rather than 1000Mbps (1.0 Gbps). When I use an old laptop on the same Ethernet cable(cat6) its running succesfully at 1.0 Gbps. Means there's no problem with the setup, router nor the cable. I tried changing the speed & duplex from auto negotiation to 1.0 Gbps but its still running at 100.0 Mbps. I also have the latest driver update for my NIC.
I think im missing something but what ? What could be the problem ?