I have a rather strange problem:
[admin@router2] > /interface ethernet export
jan/01/1970 00:01:33 by RouterOS 3.10
software id = PL88-PTT
/interface ethernet
set 0 arp=enabled auto-negotiation=yes comment="" disabled=no full-duplex=yes mac-address=00:0C:42:21:20:70 mtu=1500 name=ether1 speed=100Mbps
set 1 arp=enabled auto-negotiation=no comment="" disabled=no full-duplex=yes mac-address=00:0C:42:21:20:71 mtu=1500 name=ether2 speed=10Mbps
set 2 arp=enabled auto-negotiation=no comment="" disabled=no full-duplex=yes mac-address=00:0C:42:21:20:72 mtu=1500 name=ether3 speed=10Mbps
[admin@router2] > /interface ethernet monitor 1 once
status: link-ok
auto-negotiation: disabled
rate: 10Mbps
full-duplex: yes
[admin@router2] > /interface ethernet monitor 2 once
status: link-ok
auto-negotiation: done
rate: 1Gbps
full-duplex: yes
Notice how the configuration for ether2 and ether3 is the same, yet ether2 ends up with 10Mbps as planned and ether3 ends up with 1Gbps. The interfaces are connected ether2 <=> ether2 and ether3 <=> ether3 to another RB600 with the exact same configuration (and problem).