STP Clinical & Scientific Knowledge 3 — Questions and Answers
Question 1: What is the purpose of the PortFast feature in STP?
- Speeds up root bridge election
- Allows edge ports to skip Listening and Learning states (Correct answer)
- Doubles BPDU transmission rate
- Reduces the MaxAge timer to 5 seconds
Correct answer: Allows edge ports to skip Listening and Learning states
PortFast allows access layer ports connected to end hosts to immediately transition to Forwarding, bypassing Listening and Learning.
Question 2: Which field in the Bridge ID has the highest priority during root bridge election?
- MAC address
- Port priority
- Bridge priority (Correct answer)
- System ID extension
Correct answer: Bridge priority
The bridge priority (default 32768) is compared first; the MAC address is only used as a tiebreaker.
Question 3: BPDUGuard is enabled on a PortFast port. What happens when a BPDU is received?
- The BPDU is discarded silently
- The port transitions to Blocking state
- The port is placed in err-disabled state (Correct answer)
- A new root bridge election begins
Correct answer: The port is placed in err-disabled state
BPDUGuard shuts down a PortFast port by placing it in err-disabled state if any BPDU is received.
Question 4: In RSTP (802.1w), which port role replaces the STP Blocking state ports?
- Alternate and Backup ports (Correct answer)
- Discarding and Learning ports
- Passive and Standby ports
- Edge and Non-edge ports
Correct answer: Alternate and Backup ports
RSTP introduces Alternate (replaces blocked non-designated) and Backup (provides redundancy on shared segments) port roles.
Question 5: What STP path cost value is assigned to a 1 Gbps link using the original (short) path cost method?
- 4 (Correct answer)
- 19
- 100
- 200
Correct answer: 4
Using the original 802.1D short path cost method, a 1 Gbps link has a cost of 4.
Question 6: A switch has two equal-cost paths to the root bridge through different upstream switches. How does STP break this tie?
- By comparing port speed
- By choosing the path through the switch with the lowest Bridge ID (Correct answer)
- By choosing the most recently added path
- By using round-robin load balancing
Correct answer: By choosing the path through the switch with the lowest Bridge ID
When root path cost is tied, STP selects the path through the upstream switch with the lowest Bridge ID.
Question 7: What does the STP MaxAge timer control?
- How long a port stays in the Learning state
- How long a switch stores BPDU information before discarding it (Correct answer)
- The maximum age of MAC table entries
- How often the root bridge sends Hello BPDUs
Correct answer: How long a switch stores BPDU information before discarding it
MaxAge (default 20 seconds) is the maximum time a switch retains BPDU information before considering the root bridge unreachable.
STP Clinical & Scientific Knowledge 3
What is the purpose of the PortFast feature in STP?