I have a 25m CAT 5e cable and I'm trying to connect to a router. The connection obtained is always 100Base-TX full-duplex, it never reaches 1000Base-T. What can be the problem? As long as it says it's full duplex I assume the jacks are ok.
Ron hit the nail on the head for the most likely cause. As long as the router has a gig interface and the computer or switch connected to it is capable of gig speeds, then it's most likely a cabling issue. I had this same issue when I did some home wiring a few months ago, turned out 7 of the 8 wires were connected, not 8/8 = not 1000BaseT.
Did you try to manually enter the speed, instead of auto-negotiation?
Try this under the switchport:
Switch(config-if)#speed 1000 Switch(config-if)#duplex full
On the host, if it is a Windows machine, right-click the adapter, click properties, then 'Configure' and then 'Advanced'. Click on 'Link Speed & Duplex' and under Value, choose 1.0 Gbps Full Duplex'