Which of the following will generate an RSTP topology change notification? (Select the best answer.) *

A.
an edge port that transitions to the forwarding state

B.
a non-edge port that transitions to the blocking state

C.
a non-edge port that transitions to the forwarding state

D.
an edge port that transitions to the blocking state

E.
any port that transitions to the blocking state

F.
any port that transitions to the forwarding state

Explanation:

The IEEE 802.1D Spanning Tree Protocol was designed to keep a switched or bridged network loop free, with adjustments made to the network topology dynamically. A topology change typically takes 30 seconds, where a port moves from the Blocking state to the Forwarding state after two intervals of the Forward Delay timer. As technology has improved, 30 seconds has become an unbearable length of time to wait for a production network to failover or "heal" itself during a problem.

Topology Changes and RSTP
Recall that when an 802.1D switch detects a port state change (either up or down), it signals the Root Bridge by sending topology change notification (TCN) BPDUs. The Root Bridge must then signal a topology change by sending out a TCN message that is relayed to all switches in the STP domain. RSTP detects a topology change only when a nonedge port transitions to the Forwarding state. This might seem odd because a link failure is not used as a trigger. RSTP uses all of its rapid convergence mechanisms to prevent bridging loops from forming. Therefore, topology changes are detected only so that bridging tables can be updated and corrected as hosts appear first on a failed port and then on a different functioning port.

When a topology change is detected, a switch must propagate news of the change to other switches in the network so they can correct their bridging tables, too. This process is similar to the convergence and synchronization mechanism-topology change notification (TCN) messages propagate through the network in an ever-expanding wave.