Cisco CCNA Spanning Tree Protocol (STP) 4 — Questions and Answers
Question 1: What does the STP BPDU Guard feature do when it detects a BPDU on a PortFast-enabled port?
- It immediately puts the port into err-disabled state (Correct answer)
- It blocks the port and notifies the root bridge
- It disables PortFast on that port and runs normal STP
- It forwards the BPDU to the root bridge for analysis
Correct answer: It immediately puts the port into err-disabled state
BPDU Guard places the port in err-disabled state when a BPDU is received, protecting the network from unauthorized switches.
Question 2: In a network running PVST+, how many STP instances are maintained?
- One per VLAN (Correct answer)
- One for all VLANs combined
- One per trunk link
- One per switch
Correct answer: One per VLAN
PVST+ (Per-VLAN Spanning Tree Plus) maintains a separate STP instance for each VLAN, allowing different root bridges per VLAN.
Question 3: Which STP enhancement prevents a port from becoming a designated port if it stops receiving BPDUs, protecting against unidirectional link failures?
- Loop Guard (Correct answer)
- Root Guard
- BPDU Guard
- PortFast
Correct answer: Loop Guard
Loop Guard places a port in a loop-inconsistent blocking state if BPDUs stop arriving, preventing it from incorrectly transitioning to Forwarding.
Question 4: A switch administrator wants to ensure a specific switch always becomes the root bridge for VLAN 1. Which command achieves this?
- spanning-tree vlan 1 priority 4096
- spanning-tree vlan 1 root primary (Correct answer)
- spanning-tree vlan 1 cost 1
- spanning-tree mode rapid-pvst
Correct answer: spanning-tree vlan 1 root primary
The 'spanning-tree vlan 1 root primary' command sets the bridge priority to ensure this switch wins the root bridge election.
Question 5: What is the maximum number of hops allowed in an MST (Multiple Spanning Tree) region before a BPDU is discarded?
- 20 (Correct answer)
- 7
- 15
- 32
Correct answer: 20
MST uses a max hops field (default 20) instead of the traditional max age timer to limit BPDU propagation within a region.
Question 6: Which STP port state learns MAC addresses but does NOT forward data frames?
- Learning (Correct answer)
- Listening
- Blocking
- Forwarding
Correct answer: Learning
The Learning state populates the MAC address table but does not forward user data frames, reducing flooding when the port transitions to Forwarding.
Question 7: When using MST, what is the name of the internal spanning tree instance that interacts with external 802.1D/RSTP regions?
- IST (Internal Spanning Tree)
- CIST (Common and Internal Spanning Tree) (Correct answer)
- MSTi (MST instance)
- PVST+ instance
Correct answer: CIST (Common and Internal Spanning Tree)
The CIST (Common and Internal Spanning Tree) spans all MST regions and 802.1D bridges, representing the overall spanning tree viewed externally.
Cisco CCNA Spanning Tree Protocol (STP) 4
What does the STP BPDU Guard feature do when it detects a BPDU on a PortFast-enabled port?