After some partition shuffling, I've got a problem on my 11.10 system. The wired ethernet interface fails to come up, although the wireless interface is working. I'm using NetworkManager. I thought this may be a problem with NetworkManager, so I checked the config files, which look ok. I then tried re-installing the package, but this didn't resolve the issues. I'm not sure at this point if the problem is due to software configuration, or a hardware problem. I've also tried the cable in other router ports, but same problem.

The symptoms are:-

System settings -> Network reports that the cable is unplugged (it isn't)

ifconfig reports the following

eth0      Link encap:Ethernet  HWaddr f0:4d:a2:a2:a7:fe  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:10 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:792 (792.0 B)  TX bytes:0 (0.0 B)
          Interrupt:46 Base address:0xe000

My /etc/network/interfaces file has the following:

auto lo
iface lo inet loopback

My /etc/resolv.conf file has the following:

# Generated by NetworkManager

The router's red light is on for this port

dmesg reports ADDRCONF(NETDEV_UP): eth0: link is not ready