Hi,
Could someone give ideas as to why my MikroTik RB750G would change its ethernet connection from 1Gbps to 100Mbps, causing all my network connections to drop instantly? I’ve now gone and forced it to 100Mbps 
Not the best solution, but have you tried removing the auto-negotiation, and setup speed manually?
/interface ethernet set x speed =
In order to link at 1Gbps auto-negotiation must be enabled, as there are more parameters to negotiate other that speed and duplex mode.
How long and what kind of cable are you using? What ROS version? What device are you linking to? Does this affects all ethernet ports?
thanks kirshteins. I’ll try to do some more comprehensive tests tonight.
Its a RB750G with ROS 4.4 and it was set to auto-negotiate. I’ve tried forcing it to 1Gbps, but then I weren’t able to link at all.
Unfortunately I’m not that clued up on the cables. Its a 8 strand UTP cable that I was using for 100mbps over the last few years.
Cable quality might not be good enough or maybe it’s too long.
Do you have RB750G on both sides?
No, its connected to a PC with gigabit LAN.
The total cable length is probably 15m. On both ends I have Ethernet wall sockets, for neat cable routing.
So yes, it could simply be a matter of the cable quality not being good enough.
it should be written somewhere (usually is) what type of cable it is, for 1GBit link you need CAT 5e or CAT 6 cable.
Also make sure you’re using the T-568A or T-568B standard. You can run into interference problems on cables much shorter than 100m if you aren’t using the proper pairs.