That is definitely spanning tree kicking in. Assigning the lower priority to the core switch is a good idea. Typically your root switch will be the one that plugs into the firewall. Setting the priority on the core switch to 4096, will make it the root switch. Then as you work your way through the different layers of the network, increase the priority. Access layer switches can be left at default.  I would also double check physical connections for any loops. In situations where you have multiple connections from switch to switch, be sure those ports are configured in a LAG.  Keep us posted.
Hi, I wondering if anyone can help me, I'm a humble sysadmin with no real networking admin training.  I have the following working setup in an existing rack at our datacentre.    2x Dell Sonicwall NSA2400   2x stacked Dell Powerconnect N5524 switches    One Sonicwall is plugged into each switch (active-passive)    Each Dell server has 2x nics in bonded failover mode, one of each nic plugged into each switch This is done to allow full redundancy for NICs, cable, switches and firewall. It works well. The rack mentioned above is now full and we have ordered a new rack which is in another datacentre building at the same site. We decided to buy another set of stacked switches and _extend_ (not stack) these via SPF optics to the other rack. The idea being that we still have the switch fault tollerance but link to two racks via 10Gb. The new switches are 2x Dell Powerconnect N2048 and have been setup stacked with no devices plugged in.  I set up the SPF ports on both switches to be a trunk ports and connected the two sets of switches up, unfortunatly our site went offline, our alerting went ballistic and no servers could talk to each other !!  I quickly unplugged the new switches and after a worrying 10 seconds the alerts cleared and the site came back online.  I'm not sure why this happened and have been looking at the STP settings which from my googleing seem fine. I'm sure i'm missing something basic.  I'll post a diagram and setting in a followup post  
Issue adding new set of stacked switches as extension to network into another 
datacentre building