STP Network Design and Implementation 2 — Questions and Answers
Question 1: In a redundant network design, which STP feature allows ports to bypass the Listening and Learning states and go directly to Forwarding?
- PortFast (Correct answer)
- BPDU Guard
- Loop Guard
- Root Guard
Correct answer: PortFast
PortFast skips the Listening and Learning states, allowing a port to transition immediately to Forwarding — intended for access ports connected to end devices.
Question 2: A network engineer is designing a campus network with three distribution switches. Which STP enhancement should be deployed to ensure only designated root bridges can exist in specific VLANs?
- Root Guard (Correct answer)
- BPDU Guard
- BPDU Filter
- Loop Guard
Correct answer: Root Guard
Root Guard enforces the root bridge placement by blocking ports that receive superior BPDUs, preventing unexpected switches from becoming the root.
Question 3: Which topology creates the highest risk of STP failure due to unidirectional link failures?
- Fiber optic links without Loop Guard (Correct answer)
- Copper Ethernet links without PortFast
- Wireless bridges without BPDU Filter
- VLAN trunks without Root Guard
Correct answer: Fiber optic links without Loop Guard
Fiber optic links can fail in one direction only (receive or transmit), causing STP to not detect the failure; Loop Guard mitigates this by moving the port to a loop-inconsistent state.
Question 4: In a hierarchical three-tier network design, where should the STP root bridge ideally be placed?
- Core layer (Correct answer)
- Distribution layer
- Access layer
- DMZ switches
Correct answer: Core layer
Placing the root bridge at the core layer ensures that traffic flows optimally from access to distribution to core, avoiding suboptimal paths.
Question 5: A switch receives a BPDU on a PortFast-enabled port. If BPDU Guard is also enabled globally, what happens?
- The port is placed in errdisable state (Correct answer)
- The port transitions to Blocking state
- STP is disabled on that port
- The BPDU is silently dropped
Correct answer: The port is placed in errdisable state
BPDU Guard shuts down the port (errdisable) immediately upon receiving any BPDU, protecting the network from rogue switches on access ports.
Question 6: 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 (Correct answer)
- Enabling BPDU Filter on all trunk ports
- Setting all switches to the same bridge priority
- Using TCN BPDUs exclusively
Correct answer: Keeping the network diameter to 7 hops or fewer
STP's default timer calculations assume a maximum network diameter of 7 switches; exceeding this can cause timing issues and topology instability.
Question 7: When implementing Rapid STP (802.1w) in a network design, which port role replaces the classic STP Blocking port for faster failover?
- Alternate port (Correct answer)
- Backup port
- Designated port
- Non-designated port
Correct answer: Alternate port
In RSTP, the Alternate port provides a pre-calculated backup path to the root and can transition to Forwarding immediately when the Root port fails.
STP Network Design and Implementation 2
In a redundant network design, which STP feature allows ports to bypass the Listening and Learning states and go directly to Forwarding?