- A
Bridge ID: A combination of bridge priority and MAC address used to elect the root bridge.
The Bridge ID is an 8-byte value consisting of a 2-byte bridge priority and a 6-byte MAC address. It is used in the root bridge election process; the switch with the lowest Bridge ID becomes the root bridge.
- B
Port Cost: A value assigned to each port based on its speed, used to determine the root port on a non-root bridge.
Why wrong: This is incorrect because Port Cost is used to determine the best path to the root bridge, but it is not used to determine the root port on a non-root bridge; the root port is the port with the lowest path cost to the root bridge. The statement is partially correct but misstates the exact function.
- C
Designated Port: The port on a switch that has the lowest path cost to the root bridge on a given segment.
Why wrong: This is incorrect because the designated port is the port on a segment that has the lowest path cost to the root bridge, but it is not necessarily on the switch with the lowest path cost; it is the port that provides the best path from that segment to the root bridge. The description is ambiguous and could be confused with the root port.
- D
Blocking State: A port state in STP where the port is logically disabled to prevent loops, but can still receive BPDUs.
Why wrong: This is incorrect because in the blocking state, the port does not send or receive data frames, but it does listen for BPDUs. However, the statement says 'can still receive BPDUs' which is true, but the primary function is to prevent loops by not forwarding traffic. The description is mostly accurate but the key function is loop prevention, not just receiving BPDUs.
Quick Answer
The answer is Bridge ID, which is a combination of bridge priority and MAC address used to elect the root bridge. This works because STP requires a single, unambiguous reference point for all path calculations, and the lowest Bridge ID—determined first by priority (in increments of 4096) and then by MAC address—wins the election. On the CCNA 200-301 v2 exam, this concept often appears in drag-and-drop or multiple-choice questions asking you to match terms like Bridge ID, Root Port, PortFast, and BPDU Guard to their functions. A common trap is confusing the root bridge election with the root port selection; remember that the root bridge is elected globally by Bridge ID, while the root port is chosen locally on non-root switches by lowest path cost. For a quick memory tip, think of Bridge ID as the “birth certificate” of a switch—priority is the last name, MAC is the unique first name, and the lowest combined value becomes the root.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. A key principle to apply: the root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Match each STP-related term or feature to its most accurate function.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Bridge ID: A combination of bridge priority and MAC address used to elect the root bridge.
In STP, the root bridge is elected as the central reference switch for path calculation. The root port on a non-root switch is the port with the lowest cost path toward the root bridge. PortFast allows an edge port to immediately transition to the forwarding state, bypassing listening and learning. BPDU Guard protects the network by placing an edge port into errdisable state if a BPDU is received, preventing accidental loops.
Key principle: The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Bridge ID: A combination of bridge priority and MAC address used to elect the root bridge.
Why this is correct
The Bridge ID is an 8-byte value consisting of a 2-byte bridge priority and a 6-byte MAC address. It is used in the root bridge election process; the switch with the lowest Bridge ID becomes the root bridge.
Related concept
The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.
- ✗
Port Cost: A value assigned to each port based on its speed, used to determine the root port on a non-root bridge.
Why it's wrong here
This is incorrect because Port Cost is used to determine the best path to the root bridge, but it is not used to determine the root port on a non-root bridge; the root port is the port with the lowest path cost to the root bridge. The statement is partially correct but misstates the exact function.
- ✗
Designated Port: The port on a switch that has the lowest path cost to the root bridge on a given segment.
Why it's wrong here
This is incorrect because the designated port is the port on a segment that has the lowest path cost to the root bridge, but it is not necessarily on the switch with the lowest path cost; it is the port that provides the best path from that segment to the root bridge. The description is ambiguous and could be confused with the root port.
- ✗
Blocking State: A port state in STP where the port is logically disabled to prevent loops, but can still receive BPDUs.
Why it's wrong here
This is incorrect because in the blocking state, the port does not send or receive data frames, but it does listen for BPDUs. However, the statement says 'can still receive BPDUs' which is true, but the primary function is to prevent loops by not forwarding traffic. The description is mostly accurate but the key function is loop prevention, not just receiving BPDUs.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Bridge ID: A combination of bridge priority and MAC address used to elect the root bridge.Correct answer▾
Why this is correct
The Bridge ID is an 8-byte value consisting of a 2-byte bridge priority and a 6-byte MAC address. It is used in the root bridge election process; the switch with the lowest Bridge ID becomes the root bridge.
✗Port Cost: A value assigned to each port based on its speed, used to determine the root port on a non-root bridge.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that Port Cost determines the root port, not the designated port. The root port is the port with the lowest cost to the root bridge.
Why candidates choose this
Candidates pick this because they confuse the role of port cost in root port selection versus designated port selection.
✗Designated Port: The port on a switch that has the lowest path cost to the root bridge on a given segment.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the designated port is the port on a segment that offers the best path to the root bridge, not the port with the lowest path cost on the switch itself.
Why candidates choose this
Candidates pick this because they mix up the definitions of designated port and root port.
✗Blocking State: A port state in STP where the port is logically disabled to prevent loops, but can still receive BPDUs.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the blocking state's primary purpose is to prevent loops by not forwarding frames, not just to receive BPDUs. The statement is incomplete.
Why candidates choose this
Candidates pick this because they know that blocking state prevents loops, but they may overemphasize the BPDU reception aspect.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Be careful not to confuse the roles of root port and designated port. The root port is the port on a non-root bridge with the lowest cost to the root bridge, while the designated port is the port on a segment with the lowest cost to the root bridge. Also, remember that the Bridge ID is priority + MAC, not just priority.
Trap categories for this question
Similar concept trap
This is incorrect because the designated port is the port on a segment that has the lowest path cost to the root bridge, but it is not necessarily on the switch with the lowest path cost; it is the port that provides the best path from that segment to the root bridge. The description is ambiguous and could be confused with the root port.
Detailed technical explanation
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 network protocol designed to prevent loops in Ethernet networks by creating a loop-free logical topology. The root bridge is the central reference point in the STP topology, elected based on the lowest bridge ID. All switches calculate their best path to the root bridge, and ports are assigned roles such as root port, designated port, or blocked port to maintain a loop-free environment. STP dynamically blocks redundant paths while allowing a single active path between any two network devices. A root port is the single port on a non-root switch that provides the lowest-cost path back to the root bridge. PortFast is a Cisco feature applied to edge ports that immediately transitions the port to the forwarding state, bypassing the usual listening and learning states to speed up device connectivity. BPDU Guard is a security feature that disables a PortFast-enabled port if it receives a Bridge Protocol Data Unit (BPDU), protecting the network from potential loops caused by unauthorized switches or devices. A common exam trap is confusing the roles and functions of STP terms, such as assuming PortFast ports participate in STP calculations or that BPDU Guard is optional for all edge ports. In practice, PortFast ports do not send BPDUs and should only be enabled on ports connected to end devices. BPDU Guard protects the network by shutting down ports that receive unexpected BPDUs, preventing potential topology changes or loops. Understanding these operational roles is critical for correctly answering STP-related questions on the CCNA exam.
KKey Concepts to Remember
- The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.
- A non-root switch selects one root port as its lowest-cost path back to the root bridge to forward traffic toward the network core.
- PortFast immediately transitions an edge port to the forwarding state, bypassing STP listening and learning states to speed up host connectivity.
- BPDU Guard disables a PortFast-enabled edge port if it receives a BPDU, protecting the network from potential loops caused by unauthorized devices.
- STP prevents Layer 2 loops by blocking redundant paths and allowing only one active forwarding path between switches.
- Root ports are always in a forwarding state and represent the best path toward the root bridge on non-root switches.
- PortFast should only be enabled on ports connected to end devices, never on ports connecting to other switches.
- BPDU Guard helps maintain network stability by shutting down ports that receive unexpected BPDUs, preventing topology changes.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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Review the root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology..
What is the correct answer to this question?
The correct answer is: Bridge ID: A combination of bridge priority and MAC address used to elect the root bridge. — In STP, the root bridge is elected as the central reference switch for path calculation. The root port on a non-root switch is the port with the lowest cost path toward the root bridge. PortFast allows an edge port to immediately transition to the forwarding state, bypassing listening and learning. BPDU Guard protects the network by placing an edge port into errdisable state if a BPDU is received, preventing accidental loops.
What should I do if I get this 200-301 question wrong?
Review the root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
The root bridge is elected as the switch with the lowest bridge ID and serves as the central reference point in the STP topology.
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Last reviewed: May 17, 2026
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