RB750 and ethernet loopback

Hi!

I am having some problems when trying to do this with a RB750:
http://gregsowell.com/?p=2093
http://sites.google.com/site/kalman/tech-break/ethernetrj-45loopback

When plug the loopback on ethernet port anything changes.

Works fine in RB433 and RB493(AH). Just having problems with RB750.

Any idea?

's
Ismael