STP and Rapid PVST+ Basics

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Plain-English explanation

Redundant links between switches prevent single failures — but without Spanning Tree Protocol (STP), loops cause broadcast storms, MAC table instability, and duplicate frames. STP builds a loop-free logical tree by blocking redundant paths.

Rapid PVST+ (RPVST+) is Cisco's per-VLAN rapid STP — faster convergence than classic STP using proposal/agreement instead of waiting full timers.

Deep dive

Without STP, redundant switch links cause broadcast storms.
Supplementary figure from Panagiss CCNAmd

Root, designated, and blocking port roles in a loop-free tree.
Supplementary figure from Panagiss CCNAmd

STP Vs RSTP States — Classic STP vs RSTP port states.
From study charts · jdepew88 CCNA notes

Span Tree Comp — Rapid PVST+ port state progression.
From study charts · jdepew88 CCNA notes

STP Cost — Default STP port cost by link speed.
From study charts · jdepew88 CCNA notes

STP Vs PVST Vs RSTP Vs RPVST Vs MSTP — STP variants compared — know PVST+ and RPVST+ for CCNA.
From study charts · jdepew88 CCNA notes

Additional topic reference

Spanning Tree Protocol — Panagiss CCNAmd notes

Root bridge election: Lowest bridge priority (default 32768 + VLAN ID in PVST+) wins; tie-break lowest MAC.

Port roles:

| Role | Meaning | |------|---------| | Root | Best path toward root for the switch | | Designated | Best path onto a segment — forwards | | Alternate / Backup | Blocked redundant path |

Classic STP timers: Hello 2s, forward delay 15s, max age 20s — slow convergence.

RPVST+: Per-VLAN instances, faster transitions, still know BPDU basics for CCNA.

Port states (classic): Blocking → Listening → Learning → Forwarding (Disabled). RPVST+ uses discarding/learning/forwarding — know exams may reference either conceptually.

Step-by-step — predict blocked port

Triangle: SW-A, SW-B, SW-C fully meshed. Root is SW-A (lowest priority).

All links toward root from B and C become root or designated where appropriate
One link between B–C (or slowest path to root) becomes blocked
If root fails, blocked port transitions to forwarding — convergence event

Draw the tree before looking at show spanning-tree.

Commands to know

STP verification

show spanning-tree show spanning-tree vlan 10 show spanning-tree root

Influence root election

spanning-tree vlan 10 priority 4096 spanning-tree vlan 10 root primary

Troubleshooting

| Symptom | Likely STP-related cause | |---------|---------------------------| | Broadcast storm | Loop — STP off, BPDU filter, or uni-directional link | | Link up but not forwarding | Blocking/alternate — check spanning-tree | | Slow failover | Classic STP timers — consider Rapid PVST+ | | VLAN-specific outage | Per-VLAN STP root different — check per VLAN |

Never disable STP to "fix" slowness without removing physical loops.

Exam relevance

Exam trap

Root port is the best inbound port toward root — not "port on root bridge." Root bridge ports are all designated.

PortFast note

PortFast on access ports skips STP wait for end hosts — never on trunks between switches.

Practice checklist

Given priorities and MACs, elect root bridge
Mark root, designated, and blocked ports on a three-switch diagram
Compare classic STP vs RPVST+ convergence in one paragraph
Run show spanning-tree vlan 1 on a lab switch and interpret
Relate STP blocked ports to redundant uplink design

What is the primary purpose of Spanning Tree Protocol?

Which bridge becomes the STP root by default?

Learning objectives