Why is the link between my switch and my router always negotiating half-duplex mode?

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http://serverfault.com – I have a Cisco 2950 switch which has one of its ports connected to an Internet router provided by my ISP; I have no access to the router configuration, but I manage the switch. If I leave all switch ports with their default setup (auto-negotiation of speed and duplex mode), this link always connects at 100 MBit/s, but in half-duplex mode. I've tried replacing the cable, and also moving the link to another switch port: the result is always the same. A different device connected to the same port (or to any switch port, really) shows no problem at all. It could be guesed that someone configure (HowTos)