Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
If the links aren't actually dropping, you may just be losing the hello packets within a burst of other traffic. Do the ports indicate any drops?
What kind of link is this? I assume Ethernet, as you mention using 3850s, but will the link indicate a link down to both sides? If it does, default times should be fine. Only if the link doesn't indicate link down to both sides do you need to depend on the routing protocol's loss of neighbor timers.
If that's what's happening, you either need to increase the overall dead time interval (this to try to get some hellos outside a burst event), or you need to increase the number of hello packets within the same time interval (this to try to have some hello get through by having more [in your case fast hellos, or, if supported on the 3850, BPD]), or you use QoS to insure hello packets are protected against other traffic during burst events.
