I saw that all ethernet interfaces on one of mikrotiks are going down, and getting up after one second.
I use ROS 6.15 on RB751G-2HnD. I’ve attached log screen.
Somebody have any ideas what can cause such a problem?
It worked ok week before, it begun just today 
Hi there.
Had the same problem.
You should test cable with cable tester.
One of the wires might be broken.
OR you can replace the cable with new one instead.
Should fix the issue
–EDIT–
Might also be interference?
i’ve already tested 3 of 5. All seems to be ok.
I try to downgrade to 6.14. Seems to be ok now 
I was wrong.
Downgrade didn’t help and all cables are ok
you can also try to downgrade/upgrade firmware.
/system routerboard upgrade
btw I also thought my problem was solved. But ether1 still does it at random times.
I’ve opened support ticket with that question. Lets wait for official answer
Another cause of this is can be RFI.
Seen it with a radio transmitter with some increased SWR having its antenna cable near the ethernet cable.
Link did go down on radio Tx. And it was just a puny 4W CB radio. Tuning the antenna sorted all out.
Just to keep that in mind…
do you use FTP cable ?
I use UTP indoors, and FTP in outdoors connections. This router is more switch than rouer - it have 5 UTP links to another mikrotiks, who works like ap’s.
A typical reason is “Auto negotiate ON”. I even had the case, that the Ethernet-link between two NT-routers of same type went up and down, because they could not always agree upon the link speed to be used.
So you might try to set both sides of the links affected to 100MBit, half dplx. And see, what happens.
Thank you reinerotto. Thanks to you I solved the problem with the printer losing printouts after leaving the standby mode.
Andy