Cisco CCNA Preparation · lesson 14 of 36 · 25 min
The Problem STP Solves
Redundant switch links are essential for fault tolerance — but they create switching loops. Without loop prevention, broadcast frames circulate endlessly (broadcast storm), MAC address tables flip-flop, and the network collapses. Spanning Tree Protocol (STP) solves this by logically blocking redundant paths while keeping them available for failover.
STP Versions
STP Port Roles and States
Root Bridge Election
STP elects a root bridge — the reference point for all path calculations. The switch with the lowest Bridge ID wins. Bridge ID = Priority (default 32768) + VLAN ID + MAC address. To make a specific switch the root bridge, lower its priority:
spanning-tree vlan 1 priority 4096
Or use the macro: spanning-tree vlan 1 root primary (sets priority to 24576).
PortFast and BPDU Guard
Flashcard Drill
STP Spanning Tree Protocol — prevents switching loops by blocking redundant paths. Root bridge The switch with the lowest Bridge ID — reference point for all STP path calculations. Bridge ID Priority (default 32768) + VLAN ID + MAC address — lowest wins root bridge election. Root port Best path toward the root bridge — one per non-root switch. Designated port Best port on each segment toward the root — forwards traffic. Blocked port Redundant path blocked to prevent loops — available for failover. RSTP (802.1w) Rapid STP — converges in 1–6 seconds vs 30–50 for original STP. Rapid PVST+ Cisco default — rapid per-VLAN STP with fast convergence. PortFast Skips STP listening/learning — port goes directly to forwarding. Use on access ports only. BPDU Guard Shuts down a PortFast port if a BPDU is received — prevents rogue switches.
Knowledge Check
Spanning Tree — 10 Questions
What problem does STP solve?
- IP address conflicts
- Switching loops and broadcast storms
- VLAN mismatches
- Routing loops
Answer: Switching loops and broadcast storms
STP prevents switching loops caused by redundant paths, which would otherwise cause broadcast storms.
Which switch wins the root bridge election?
- Highest priority
- Lowest Bridge ID
- Most ports
- Highest MAC address
Answer: Lowest Bridge ID
The switch with the lowest Bridge ID (priority + VLAN + MAC) becomes the root bridge.
Which STP version is the default on modern Cisco switches?
- 802.1D STP
- RSTP
- PVST+
- Rapid PVST+
Answer: Rapid PVST+
Rapid PVST+ is the default on modern Cisco switches — per-VLAN with fast convergence.
The STP port role that provides the best path toward the root bridge is called the _____ port.
Answer: root
Each non-root switch has one root port — the port with the best path to the root bridge.
How long does original 802.1D STP take to converge?
- 1–6 seconds
- 10–15 seconds
- 30–50 seconds
- 2 minutes
Answer: 30–50 seconds
Original 802.1D STP takes 30–50 seconds to converge — this is why RSTP was developed.
PortFast should only be configured on:
- Trunk ports
- Root ports
- Access ports connected to end devices
- All switch ports
- Answer: Access ports connected to end devices
PortFast skips STP states — only safe on ports connected to end devices, never on switch-to-switch links. BPDU Guard protects against:
- Broadcast storms
- Rogue switches being connected to PortFast ports
- Root bridge failures
- VLAN mismatches
- Answer: Rogue switches being connected to PortFast ports
BPDU Guard shuts down a PortFast port if a BPDU is received — prevents unauthorized switches.
A blocked STP port can forward traffic if the designated port fails.
- True
- False
Answer: True
Blocked ports are available for failover — if the active path fails, STP unblocks the redundant port.
To make a switch the root bridge, you should:
- Increase its priority
- Lower its priority
- Increase its MAC address
- Add more ports
- Answer: Lower its priority
Lower Bridge ID wins the root election. Lowering priority (e.g. to 4096) makes the switch win.
The default STP bridge priority is _____.
Answer: 32768
Default priority is 32768. Must be set in multiples of 4096.
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