Correct Answer: D
Explanation/Reference:
Explanation:
Since STP does not implement any authentication or encryption to protect the exchange of BPDUs, it is vulnerable to unauthorized participation and attacks. Cisco IOS offers the STP Root Guard feature to enforce the placement of the root bridge and secure the core of the STP domain. STP root guard forces a port to become a designated port so that no switch on the other end of the link can become a root switch. If a port configured for root guard receives a superior BPDU, the port it is received on is blocked. In this way, STP root guard blocks other devices from trying to
become the root bridge.
STP root guard should be enabled on all ports that will never connect to a root bridge, for example, all end user ports. This ensures that a root bridge will never be negotiated on those ports.
Reference.
http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Security/Baseline_Security/securebase book/ sec_chap7.html