I'm trying to resolve an STP problem which has taken our whole network down the last two days, and I want to make sure I have the ideal MSTP configuration on our Catalyst 6509 for interoperating with all our switches, especially Meraki but also some legacy HP.   Initially I had this configuration on our core switch when I transitioned it from PVST to MSTP:   spanning-tree mode mst spanning-tree extend system-id spanning-tree mst 0-1 priority 8192 spanning-tree vlan 1-4094 priority 8192   We're a school district, and I set this up during the summer when very few staff were in the buildings. Reading up on  MSTP there were suggestions about creating regions and instances, so I added this:   spanning-tree mst configuration name region1 instance 1 vlan 3-4, 6-8, 12-16, 100, 110, 120, 200, 300, 999-1000   I didn't see effects, positive or negative, to adding this. Are they necessary or helpful in any way, or should I just remove the region and instance to the core switch?   For further mitigation today I added "spanning-tree guard root" to all ports on the Catalyst 6509 and added this to the config:   spanning-tree portfast edge bpduguard default     I'm hoping this will help prevent spanning tree from going totally haywire again. ... View more