Innanzitutto un grazie doveroso per le risposte

Purtroppo il problema persiste.
@ Gomna
/etc/network/interfaces gia' contiene :
allow-hotplug eth0
@DrBrex
ho installato ifplugd, ma neanche cio' porta benefici.
Questo e' quello che vedo in dmesg (boot con cavo scollegato)
eth0: RTL8168b/8111b at 0xf8840000, 00:18:f3:b1:c5:3d, IRQ 177
r8169: eth0: link down
ADDRCONF(NETDEV_UP): eth0: link is not ready
e questo e' cio' che mi restituisce ifconfig:
eth0 Link encap:Ethernet HWaddr 00:18:F3:B1:C5:3D
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:6 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:468 (468.0 b) TX bytes:0 (0.0 b)
Interrupt:177
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:112 errors:0 dropped:0 overruns:0 frame:0
TX packets:112 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:7330 (7.1 KiB) TX bytes:7330 (7.1 KiB)
Qualche altra idea?
Grazie