This last weekend I tried to replace a stack of 3750G's with a stack of new 3850's.
The stack of switches has 2 fibre uplinks to the core switches (4503-E).
It didn't go well, and I had to revert the project.
The stack was all pre-configured in the workshop but could not be tested on the live LAN. When I connected it , I first connected up 1 of the 2 uplinks. The link went to UP status, and I could see the remote core switch via CDP, but it would not learn the VLANs (VTP was setup correctly), and looking at the port on the switch, it flashed green, went solid green, then flashed amber slowly. This cycled continuously, whilst the link status remained up and no errors were logged by either switch.
I debugged spanning-tree but nothing stood out. I tried a replacement SFP, and a different slot but no difference.
I re-connected one of the old 3750's and immediately the link came up stable, and everything was fine.
I then erased the config on the new stack, and simply set the fibre port as trunk, with udld enabled (matching the remote end) (neither switch supports ISL, so the trunk is dot1q). The same issue persisted.
Last night I spent a few hours in the office investigating. I grabbed a spare 3560G from the store, configured a fibre port as a trunk, and connected this to the 3850's, and this worked fine. I then provisioned a new fibre port on the core 4503-E and connected this up using brand new fibre tails, but had exactly the same issues as described above.
The switches were running cat3k_caa-iosd-universalk9.SPA.150-1.EX3, and I have upgraded them to cat3k_caa-universalk9.SPA.03.03.01.SE.150-1.EZ1.bin as part of my troubleshooting but this has not made a difference.
Does anyone have any advice? I am starting to wish I spec'd 3750X's instead!!