QUESTION: 1
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.

Answer(s): B
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
Recal 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
QUESTION: 2
Refer to the exhibit.

2

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