STP Configuration and Management 3 — Questions and Answers
Question 1: Which STP feature automatically sets an access port to forwarding state immediately without going through listening and learning?
- BPDU Filter
- PortFast (Correct answer)
- Loop Guard
- Root Guard
Correct answer: PortFast
PortFast bypasses the listening and learning states and immediately transitions an access port to forwarding, reducing convergence time for end devices.
Question 2: What command enables PortFast on all non-trunking access ports globally on a Cisco switch?
- spanning-tree portfast default (Correct answer)
- spanning-tree portfast all
- spanning-tree portfast enable global
- interface range fastethernet 0/1-24 spanning-tree portfast
Correct answer: spanning-tree portfast default
The global command 'spanning-tree portfast default' enables PortFast on all non-trunking interfaces without configuring each one individually.
Question 3: Root Guard is best applied to which type of port?
- Ports connected to end-user devices
- Designated ports where unauthorized root bridges might appear (Correct answer)
- Root ports on non-root switches
- Trunk ports connected to the core layer
Correct answer: Designated ports where unauthorized root bridges might appear
Root Guard is applied to designated ports to prevent a connected switch from becoming the root bridge by placing the port in root-inconsistent state if a superior BPDU is received.
Question 4: What does the 'spanning-tree loopguard default' command do?
- Enables Loop Guard on all PortFast-enabled ports
- Enables Loop Guard globally on all point-to-point ports (Correct answer)
- Disables BPDUs on all ports to prevent loops
- Enables Loop Guard only on root ports
Correct answer: Enables Loop Guard globally on all point-to-point ports
The 'spanning-tree loopguard default' command globally enables Loop Guard on all point-to-point ports, protecting against unidirectional link failures.
Question 5: Which STP timer controls how long a port remains in the listening and learning states?
- Hello time
- Max age
- Forward delay (Correct answer)
- Hold time
Correct answer: Forward delay
Forward delay (default 15 seconds) is the time a port spends in each of the listening and learning states before transitioning to forwarding.
Question 6: What is the effect of reducing the STP max-age timer below its default of 20 seconds?
- Ports transition to forwarding faster after a link failure
- The root bridge sends BPDUs more frequently
- Switches declare a root bridge failure more quickly, speeding up convergence (Correct answer)
- BPDU guard trips more frequently on edge ports
Correct answer: Switches declare a root bridge failure more quickly, speeding up convergence
Reducing max-age causes switches to declare stale BPDU information invalid sooner, speeding up failure detection and convergence.
Question 7: Which command displays a summary of the number of STP instances and port states on a Cisco switch?
- show spanning-tree detail
- show spanning-tree vlan 1
- show spanning-tree summary (Correct answer)
- show spanning-tree root
Correct answer: show spanning-tree summary
The 'show spanning-tree summary' command provides a concise overview of all active STP instances and the count of ports in each state.
STP Configuration and Management 3
Which STP feature automatically sets an access port to forwarding state immediately without going through listening and learning?