Which statement is true about RSTP topology changes?
A.
Any change in the state of the port generates a TC BPDU.
B.
Only non-edge ports moving to the forwarding state generate a TC BPDU.
C.
If either an edge port or a non-edge port moves to a block state, then a TC BPDU is generated.
D.
Only edge ports moving to the blocking state generate a TC BPDU.
E.
Any loss of connectivity generates a TC BPDU.
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 non-edge 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 (TC) messages propagate through the network in an ever-expanding wave.
Reference:
CCNP BCMSN Official Exam Certification Guide, Fourth Edition, Chapter 11: Advanced Spanning
Tree Protocol, Rapid Spanning Tree Protocol, Topology Changes and RSTP, p. 269