In which state is an interface placed if root-protect is enabled and a superior BPDU is received?

When root protection is enabled on an interface and a superior Bridge Protocol Data Unit (BPDU) is received, the interface is placed in a blocking state. Root protection is a feature used to prevent a switch port from becoming the root port when receiving superior BPDUs, which could lead to topology loops.

In this situation, the interface detects a BPDU that indicates the presence of a superior switch (essentially a switch with a lower Bridge ID value), and in response, it transitions to the blocking state to protect the spanning tree topology. This transition helps maintain loop-free forwarding paths by ensuring that only designated ports that are not impacted by this superior BPDU continue to forward traffic, while the blocking interface prevents any potential loops by not forwarding frames until the conditions change (e.g., when it receives a non-superior BPDU or when root protection is cleared).

Thus, in the case of a superior BPDU being received with root protection enabled, the action taken is to block the interface, ensuring that the spanning tree remains stable and loop-free.