Behaviour of eth0 not understood

[Charles Trois]

On my (new style) iMac, with YDL 2.3, I frequently see on the log lines such
as these:

Apr 28 12:14:14 sirius kernel: eth0: switching to forced 100bt
Apr 28 12:14:21 sirius kernel: eth0: switching to forced 10bt

and this goes on repeatedly until some sort of action is taken (I have not
quite identified those actions that interrupt the process).

Is this quite normal, or is it proof of some defect or erroneous
configuration somewhere? And, in this case, what should be done?

I shall be grateful for any hints.

Charles