I recently replaced a 100Mbit switch with a gigabit one, and tested that two computers plugged directly into that switch could, indeed, share data at gigabit speeds (by the simple and obvious technique of stuffing a gigabyte of data down the wire and seeing how long it takes). All working beautifully. Fast-forward a few days, and I'm seeing that file transfers are slowish again;
ethtool eth0 on one of the systems shows that it's dropped to 100Mb/s.
ifdown eth0 and
ifup eth0 gets network speed showing 1000Mb/s, and data began flowing at roughly 20MB/s.
What could cause the network interface to be dropped to a lower speed?
Debian Wheezy on the affected system.