An administrator recently configured all ports for rapid transition using PortFast. After testing, it has been determined that several ports are not transitioning as they
should. What is the reason for this?

A.
RSTP has been enabled per interface and not globally.

B.
The STP root bridge selection is forcing key ports to remain in non-rapid transitioning mode.

C.
STP is unable to achieve rapid transition for trunk links.

D.
The switch does not have the processing power to ensure rapid transition for all ports.

Explanation:
RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links, not on trunk links. The link type is automatically derived
from the duplex mode of a port. A port that operates in full-duplex is assumed to be point-to-point, while a half-duplex port is considered as a shared port by default.
This automatic link type setting can be overridden by explicit configuration. In switched networks today, most links operate in full-duplex mode and are treated as
point-to-point links by RSTP. This makes them candidates for rapid transition to the forwarding state.
http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree- protocol/24062-146.html