Which statement is true about RSTP topology changes?
A.
Any change in the state of the port generates a TC BPDU.
B.
Only nonedge ports moving to the forwarding state generate a TC BPDU.
C.
If either an edge port or a nonedge 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 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 (TC)
messages propagate through the network in an everexpanding 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