The correct answer is that Gi1/0/24 became the root port because it has a lower root path cost to the root bridge. Spanning Tree Protocol selects the root port by evaluating the cumulative path cost from the local switch to the root bridge, and the interface with the lowest cost wins the election. In this scenario, Gi1/0/24’s path cost of 4 is significantly lower than Gi1/0/23’s cost of 19, making it the superior path toward the root, so it transitions to forwarding while Gi1/0/23 is blocked as an alternate port to prevent a loop. On the CCNA 200-301 v2 exam, this concept tests your understanding of STP root port selection logic, where path cost is the primary tie-breaker—only if costs are equal does the exam move to sender bridge ID or port ID. A common trap is assuming higher bandwidth always wins, but STP uses lower cost as better; remember the mnemonic “Low cost, no loss” to recall that the smallest path cost claims the root port.
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: a non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.. 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.
Exhibit
SW1# show spanning-tree vlan 20
VLAN0020
Root ID Priority 24596
Address 0011.2233.4455
Bridge ID Priority 32788
Address 00aa.bbcc.ddee
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/23 Altn BLK 19 128.23 P2p
Gi1/0/24 Root FWD 4 128.24 P2p
A switch unexpectedly blocks a link toward the distribution layer. Gi1/0/24 shows a path cost of 4 while Gi1/0/23 shows a path cost of 19. Why did interface Gi1/0/24 become the root port instead of Gi1/0/23?
SW1# show spanning-tree vlan 20
VLAN0020
Root ID Priority 24596
Address 0011.2233.4455
Bridge ID Priority 32788
Address 00aa.bbcc.ddee
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/23 Altn BLK 19 128.23 P2p
Gi1/0/24 Root FWD 4 128.24 P2p
A
Gi1/0/24 has a lower port number, so STP always prefers it first.
Why wrong: This is wrong because STP does not begin by choosing the lowest port number. Port identifiers are tie-breakers used only when more important values, such as path cost, are equal.
B
STP prefers interfaces with the highest path cost to reduce loops.
Why wrong: This is wrong because STP is designed to prefer the lowest-cost path, not the highest.
C
Gi1/0/23 is blocked because alternate ports are always chosen over root ports.
Why wrong: This is wrong because the root port is the preferred forwarding path toward the root bridge, while the alternate port is the backup path that remains blocked.
D
has a lower root path cost to the root bridge
Correct. The root path cost is the primary determinant for root port selection; lower cost wins, which is why Gi1/0/24 (cost 4) is preferred over Gi1/0/23 (cost 19).
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
has a lower root path cost to the root bridge
Spanning Tree chooses a root port by looking for the best path toward the root bridge. In this case, Gi1/0/24 shows a cost of 4, while Gi1/0/23 shows a cost of 19. Lower cost is better, so Gi1/0/24 is selected as the root port and moves into forwarding. Gi1/0/23 becomes an alternate port and is placed into a blocking state to prevent a loop. STP compares root path cost first; only if the cost is tied does it move on to tie-breakers like sender bridge ID and port ID. The lower cost on Gi1/0/24 explains why that port won the root-port election.
Key principle: A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Gi1/0/24 has a lower port number, so STP always prefers it first.
Why it's wrong here
This is wrong because STP does not begin by choosing the lowest port number. Port identifiers are tie-breakers used only when more important values, such as path cost, are equal.
When this WOULD be correct
In a different question scenario where the context involves a simplified STP decision-making process, if the question stated that all ports were configured with the same path cost, then the option could be correct if it asked which port would be preferred based on the lowest numerical identifier.
✗
STP prefers interfaces with the highest path cost to reduce loops.
Why it's wrong here
This is wrong because STP is designed to prefer the lowest-cost path, not the highest.
When this WOULD be correct
In a different question setup, if the question stated that STP was configured to prefer paths with higher costs for specific traffic types, then this option could be correct. For example, if the exam scenario involved a network design where certain traffic flows were intentionally routed through higher-cost paths to manage bandwidth.
✗
Gi1/0/23 is blocked because alternate ports are always chosen over root ports.
Why it's wrong here
This is wrong because the root port is the preferred forwarding path toward the root bridge, while the alternate port is the backup path that remains blocked.
When this WOULD be correct
In a different scenario where the question states that Gi1/0/23 is indeed an alternate port and that the switch is configured to prefer alternate ports over root ports, this option could be correct. For example, if the question specifies that Gi1/0/23 was previously a root port but was blocked due to a topology change, making Gi1/0/24 the new root port.
✓
has a lower root path cost to the root bridge
Why this is correct
Correct. The root path cost is the primary determinant for root port selection; lower cost wins, which is why Gi1/0/24 (cost 4) is preferred over Gi1/0/23 (cost 19).
Related concept
A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.
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.
✓has a lower root path cost to the root bridgeCorrect answer▾
Why this is correct
Correct. The root path cost is the primary determinant for root port selection; lower cost wins, which is why Gi1/0/24 (cost 4) is preferred over Gi1/0/23 (cost 19).
✗Gi1/0/24 has a lower port number, so STP always prefers it first.Wrong answer — click to see why▾
Why this is wrong here
STP does not use port number as the primary criterion; it is only a tie-breaker when path cost, bridge ID, and sender bridge ID are all equal. Here, the path costs differ, so port number is irrelevant.
★ When this WOULD be the correct answer
In a different question scenario where the context involves a simplified STP decision-making process, if the question stated that all ports were configured with the same path cost, then the option could be correct if it asked which port would be preferred based on the lowest numerical identifier.
Why candidates choose this
Students often confuse STP's tie-breaking rules, thinking lower port number always wins, but this only applies when all higher-priority values are identical.
✗STP prefers interfaces with the highest path cost to reduce loops.Wrong answer — click to see why▾
Why this is wrong here
STP is designed to select the path with the lowest total cost to the root bridge, not the highest. Choosing a higher-cost path would increase latency and waste bandwidth.
★ When this WOULD be the correct answer
In a different question setup, if the question stated that STP was configured to prefer paths with higher costs for specific traffic types, then this option could be correct. For example, if the exam scenario involved a network design where certain traffic flows were intentionally routed through higher-cost paths to manage bandwidth.
Why candidates choose this
Some might mistakenly think that higher cost implies more redundancy or loop prevention, but STP's goal is to minimize cost for optimal forwarding.
✗Gi1/0/23 is blocked because alternate ports are always chosen over root ports.Wrong answer — click to see why▾
Why this is wrong here
The root port is the forwarding port toward the root bridge, while the alternate port is a blocked backup. The alternate port is not chosen over the root port; it only becomes active if the root port fails.
★ When this WOULD be the correct answer
In a different scenario where the question states that Gi1/0/23 is indeed an alternate port and that the switch is configured to prefer alternate ports over root ports, this option could be correct. For example, if the question specifies that Gi1/0/23 was previously a root port but was blocked due to a topology change, making Gi1/0/24 the new root port.
Why candidates choose this
The terms 'alternate' and 'root' can be confusing; students may think alternate ports are preferred, but they are actually backups.
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
Remember that STP prioritizes root path cost over other factors like port numbers or bridge IDs unless there's a tie.
Detailed technical explanation
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 protocol designed to prevent switching loops in redundant network topologies by electing a root bridge and determining the best paths to it. Each non-root switch selects a single root port, which is the port with the lowest total path cost to the root bridge. Path cost is cumulative and reflects the speed of the links along the path, with lower costs indicating more efficient routes. STP uses this cost metric to ensure a loop-free topology by forwarding frames only along the best paths.
When a switch receives multiple possible uplinks toward the root bridge, it compares the root path cost advertised on each port. The port with the lowest path cost becomes the root port and is placed in the forwarding state. If two ports have the same path cost, STP uses tie-breakers such as the sender bridge ID and sender port ID to decide. Ports that are not selected as root ports but could provide alternate paths are placed in a blocking state to prevent loops.
A common exam trap is to assume that STP always prefers ports with lower interface numbers or that alternate ports are preferred over root ports. In reality, STP prioritizes the lowest root path cost first, then uses port numbers only as tie-breakers. Understanding this decision process is critical for CCNA candidates to correctly interpret STP output and troubleshoot blocked ports in Cisco networks, ensuring efficient and loop-free Layer 2 topologies.
KKey Concepts to Remember
A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.
STP calculates path cost by summing the costs of all links from the switch to the root bridge, preferring the lowest total cost.
If multiple ports have equal path cost, STP uses the sender bridge ID and sender port ID as tie-breakers to select the root port.
Ports that are not selected as root ports but provide backup paths are placed in a blocking state to prevent switching loops.
STP does not inherently prefer ports with lower interface numbers unless used as a tie-breaker after path cost and bridge ID.
The root port is always the preferred forwarding path toward the root bridge, while alternate ports remain blocked.
STP path cost values reflect link speeds, with lower costs indicating faster or more efficient links to the root bridge.
Understanding STP port roles and path cost calculations is essential for diagnosing why certain ports become root or blocked.
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
A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review a non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost..
What is the correct answer to this question?
The correct answer is: has a lower root path cost to the root bridge — Spanning Tree chooses a root port by looking for the best path toward the root bridge. In this case, Gi1/0/24 shows a cost of 4, while Gi1/0/23 shows a cost of 19. Lower cost is better, so Gi1/0/24 is selected as the root port and moves into forwarding. Gi1/0/23 becomes an alternate port and is placed into a blocking state to prevent a loop. STP compares root path cost first; only if the cost is tied does it move on to tie-breakers like sender bridge ID and port ID. The lower cost on Gi1/0/24 explains why that port won the root-port election.
What should I do if I get this 200-301 question wrong?
Review a non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A non-root switch selects one root port as its lowest-cost path back to the root bridge based on cumulative path cost.
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