We have a Meraki Switch (MS250-48P) that is set w/ an STP priority of 61440, and it has a single uplink to the Internet that is reached via an AT&T Switched Ethernet Circuit (MPLS) that extends the L2 network over to another location where the ISP DIA resides. At that location, there's a Cisco Catalyst 4500 that the Meraki connects to, and that 4500 is the root-bridge for the L2 network. The 4500 is set with a priority of 4096 for all vlan's, and has root-guard configured on every port that downlinks to another switch so that the 4500 will remain the root-bridge. Root-guard is also applied on the catalyst 4500 port that links back to the abovementioned Meraki switch over the MPLS circuit. However, at least twice a day (and sometimes more than that) the uplink port on the Meraki switch transitions from a root port to designated, and the Meraki switch becomes the root-bridge. Usually this does not last for long (less than a minute in most cases) and the switch quickly loses the root bridge designation, and the Catalyst 4500 again becomes the root bridge. To my knowledge, no other switch in the Layer 2 network is set with a better (lower) priority. My question is this: what could be causing the Meraki switch to become the root? The Physical ports on both ends of the AT&T MPLS circuit (at least the ports on our gear) are clean: no crc's, no fragmentation, no output drops, etc. Could this be due to an issue with Spanning-Tree mode incompatibility? The 4500 is running rapid-pvst and the Meraki of course is running RSTP. The inconsistent/intermittent nature of the problem makes it a bit more confusing to me, tbh. When I run the "show spanning-tree detail" command on the 4500, each vlan shows that it "is executing the rstp compatible Spanning-Tree protocol", so that sounds promising, though I'm still not sure. Any ideas, suggestions, or questions are welcome. Really turning into a headscratcher at this point, thanks for your help in advance! -Ernest
