I just put up an RB800 + XR5, I've been noticing its dropping its ethernet link, Its being powered by pacwireles 48v PoE, its connected to a Cisco2950G. I have 3 other boards 433AH on that same switch and none are dropping, has anyone else seen this happen before? I did a /system routerboard settings print and got this..
baud-rate: 115200
boot-delay: 2s
boot-device: nand-if-fail-then-ethernet
enter-setup-on: any-key
cpu-frequency: 800MHz
memory-frequency: 400MHz
boot-protocol: bootp
enable-jumper-reset: yes
force-backup-booter: no

If that helps any, I'm bridging the wlan1 with the ether1 like I'm doing with all my other boards, one thing I noticed is under bridge the L2MTU is 2290, Ether1 is 1600. Anyways if anyone has any suggestions that I can check on the unit please, I will send a climber up the tower the min. I have clear sky's but I just want to rule anything out before I send someone up to re rerun cables and what not...

Try disable auto negotiation on RB800 and set speed to 100Mbit. What cable length you use?

I have exactly the same setup; and of course the same problems. My RB800 is powered with POE (48V over datalines) and connected via 1Gbs link to a Cisco 3800.
Strangely our link is sometimes up for more than 12 hours and then suddenly drops and comes back seconds later.
I´ve disabled autoneg yesterday; Had no drops since then.

just note that 1Gbps link requires auto-negotiation.

Thank you.

The link was up with 1Gbs and is now 100Mbit without autoneg and stable - didn´t see this.
The routerboards IF was set to 100Mbits but synced with 1Gbs. Does the speed setting do anything, when autoneg is enabled?
I´ll reenable autoneg, set the speed to 1Gbs (if that does anything) and wait...;

speed setting is ignored if auto-negotiation is set. It is there so you can set 100MBps or 10MBps if needed.

Try disable auto negotiation on RB800 and set speed to 100Mbit. What cable length you use?

PoE cable length is probably 60ft max, jumper to switch 7ft, I tired taking it off auto negotiate and getting about 3% packet loss, gonna keep a ping going all day just to make sure it works. Before it would drop several times within an hr or two yesterday I lost it for about 30mins.

Try set 100Mbit half-duplex

I set the device to 100/full and had it ping a device on the same switch 10000 times, dropped 2 packets. So it seems to have solved the issue, thank you everyone for your help.

I just recently revisted this issue, I am trying to set my Cisco switch to 100/full for the port going to the RB800, the RB800 is set 100/full as well auto negotiate off. The port however on the switch reads 100/half, so I set the port on the switch to 100/full and it drops connectivity to the rb800. It wont come back until I set it 100/auto, I really want this thing at 100/full.