Network Design and Operations
With default timers, classic STP (IEEE 802.1D) takes about 30 to 50 seconds after a failure before a previously blocked port starts forwarding. Which mechanism lets RSTP switch over faster?
- AIt adds a TTL-like field to frames and discards looping frames after a set number of hops, so ports no longer need to be blocked
- BNeighboring switches exchange a proposal and an agreement to move ports to forwarding immediately, and alternate ports that can replace the root port are chosen in advance
- CIt uses all links at once to spread frames, so that if any link fails the remaining links keep carrying traffic
Show answer and explanation
Answer: B. Neighboring switches exchange a proposal and an agreement to move ports to forwarding immediately, and alternate ports that can replace the root port are chosen in advance
Classic STP moves ports to the forwarding state cautiously, waiting on timers so that no loop forms. With the default values, the max age of 20 seconds plus two forward delays of 15 seconds, one each for the listening and learning states, add up to as much as 20 + 15 × 2 = 50 seconds. RSTP has switches connected point-to-point exchange a proposal and an agreement, so a port can start forwarding without waiting for timers. It also chooses in advance an alternate port that takes over when the root port fails, so in many cases the switchover takes a few seconds or less. The basic idea of preventing loops by blocking redundant ports is the same as in classic STP.
- A. Ethernet frames have no TTL-like field, and RSTP also prevents loops by blocking redundant ports.
- B. Correct. It starts forwarding quickly through exchanges between neighbors, without waiting for timers.
- C. That is closer to the idea of link aggregation. RSTP blocks some of the redundant links.