Well, The BPDU guard feature can be globally enabled on the switch or can be enabled per interface, but the feature operates with some differences. At the global level, you enable BPDU guard on Port Fast-enabled STP ports by using the spanning-tree portfast bpduguard default global configuration command. Spanning tree shuts down STP ports that are in a Port Fast-operational state if any BPDU is received on those ports. In a valid configuration, Port Fast-enabled STP ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port signals an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the interface in the error-disabled state.   I understand that in this case you should not use BPD guard between two switches.    I would be better to use the root guard on the core switch ports that uplink with other switches.     ... View more