STP Cheat Sheet 2026

The 30 highest-yield STP facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What is the best practice for STP timer configuration? → Use default timers unless there is a specific justified reason to change them
2. How does STP handle unidirectional link failures? → Through features like UDLD (Unidirectional Link Detection) and loop guard
3. Which device role in STP is responsible for forwarding traffic to all other switches in the network segment? → Designated Port
4. A switch port remains in blocking state even though it should be forwarding. Which tool helps identify whether Root Guard or Loop Guard is the cause? → show spanning-tree inconsistentports
5. Which imaging technique uses magnetic fields? → MRI
6. What is the default Hello Time interval in 802.1D STP? → 2 seconds
7. What is the primary advantage of PVST+ over classic 802.1D STP? → It runs a separate spanning tree instance per VLAN, enabling per-VLAN load balancing
8. Which MSTP concept describes the role of an MST region boundary port connecting to another region or a legacy STP bridge? → Boundary Port
9. What does the 'spanning-tree loopguard default' command do? → Enables Loop Guard globally on all point-to-point ports
10. Which IEEE standard introduced Rapid Spanning Tree Protocol (RSTP)? → IEEE 802.1w
11. Which topology creates the highest risk of STP failure due to unidirectional link failures? → Fiber optic links without Loop Guard
12. Which feature detects unidirectional fiber link failures at the physical layer, complementing Loop Guard at the STP layer? → UDLD (UniDirectional Link Detection)
13. When configuring MSTP on Cisco switches, which command sets the MST region name? → spanning-tree mst configuration name
14. Which feature automatically selects the optimal STP port priority and cost values based on the port speed in modern Cisco implementations? → STP Auto-cost based on interface bandwidth detection
15. What is the STP behavior when two switches are connected by two parallel links and one switch is non-root? → One port becomes root port; the other is blocked
16. How often does the root bridge send BPDUs? → Every 2 seconds by default (hello timer)
17. A switch has two equal-cost paths to the root bridge through different upstream switches. How does STP break this tie? → By choosing the path through the switch with the lowest Bridge ID
18. Which condition causes a non-root switch to generate a TCN BPDU? → A port transitions from Blocking to Forwarding or from Forwarding to Blocking
19. In which STP port state does a switch port learn MAC addresses but NOT forward data frames? → Learning
20. What is the purpose of Loop Guard in STP? → To protect against loops caused by unidirectional link failures on non-designated ports
21. What STP feature prevents a port from transitioning to Forwarding if it stops receiving BPDUs from the designated switch? → Loop Guard
22. Which command is used on a Cisco switch to manually set a switch as the root bridge using a specific priority? → spanning-tree vlan 1 priority 0
23. Which STP design practice reduces convergence time in large Layer 2 networks by limiting the STP diameter? → Keeping the network diameter to 7 hops or fewer
24. What is the Bridge Protocol Data Unit (BPDU) guard feature designed to do? → Shut down a PortFast port if it receives a BPDU
25. What causes an STP topology change? → A port transitioning to or from the forwarding state
26. What determines the Root Bridge in STP? → The switch with the lowest Bridge ID
27. Which STP mechanism prevents a port from immediately transitioning to Forwarding after a link comes up? → Forward Delay timer
28. In STP, what does a Topology Change Notification (TCN) BPDU cause receiving switches to do? → Age out MAC table entries faster
29. Which STP port state actively forwards frames AND learns MAC addresses? → Forwarding
30. Which STP port state is functionally equivalent to the Discarding state introduced in RSTP (802.1w)? → Blocking