Quiz: STP / Spanning Tree

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3 questions

L0 (1 questions)

1. Why is Spanning Tree Protocol necessary on a network with redundant Layer 2 links?

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Ethernet has no TTL at Layer 2, so a broadcast frame entering a loop circulates forever, multiplying at each pass. Within seconds this causes a broadcast storm: switch CPUs saturate, MAC tables overflow and thrash, and all hosts on the VLAN become unreachable. STP prevents this by logically disabling redundant paths while keeping them available for failover.

L1 (1 questions)

1. What is PortFast and BPDU Guard, and why should they be used together on access ports?

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PortFast skips the normal STP convergence (30-50 seconds of blocking/listening/learning) on access ports connected to hosts, bringing them to forwarding immediately. BPDU Guard shuts down a PortFast-enabled port if it receives a BPDU, which indicates someone plugged in a switch (creating a potential loop). Together they provide fast host connectivity while protecting against rogue switches on access ports.

L2 (1 questions)

1. Classic STP (802.1D) takes 30-50 seconds to converge. How does RSTP (802.1w) reduce this to 1-3 seconds?

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RSTP makes three key improvements:
1. Edge ports (equivalent to PortFast) skip to forwarding immediately for host-facing ports.
2. Proposal/agreement mechanism allows switches to negotiate directly with neighbors instead of waiting through timer-based states.
3. Alternate and backup ports are pre-computed failover paths that can activate immediately without recalculating the tree. RSTP also simplifies port states from five (blocking, listening, learning, forwarding, disabled) to three (discarding, learning, forwarding).