3.5. Spanning Tree Protocol (Rapid PVST+)

💡 First Principle: Physical loops in a switched network cause broadcast storms that can bring down the entire network within seconds. Spanning Tree prevents this by logically blocking redundant paths while keeping them available for failover. Think of it as a traffic controller—it closes some roads to prevent gridlock, but can reopen them instantly if the main road fails.

Consider this disaster scenario: Someone plugs both ends of a cable into the same switch, creating a loop. A single broadcast enters the loop and circulates forever, multiplying each time it passes through. Within seconds, the switch CPU is overwhelmed, and the entire VLAN goes down. Without STP, one careless connection can crash a network. With STP, the loop is detected and blocked before the first broadcast storm.

What happens when STP is misconfigured: The root bridge election determines the entire network topology. If an old, slow switch accidentally becomes root, traffic takes suboptimal paths, latency increases, and core links may be underutilized while access links are saturated. Controlling root bridge election isn't optional—it's essential for predictable performance.

The exam focuses on PVST+ and Rapid PVST+, Cisco's per-VLAN implementations. Each VLAN runs its own STP instance, allowing different topologies for different traffic patterns.

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Why Loops Are Dangerous

Without STP, a broadcast frame on a loop would:

1. Be forwarded out all ports (except incoming)
2. Arrive on the next switch and be forwarded again
3. Continue forever, multiplying exponentially
4. Consume all bandwidth and crash the network