Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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 P2pA switch unexpectedly blocks a link toward the distribution layer. Based on the output, why did interface Gi1/0/24 become the root port instead of Gi1/0/23?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
Gi1/0/24 has a lower port number, so STP always prefers it first.
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
Distractor review
STP prefers interfaces with the highest path cost to reduce loops.
This is wrong because STP is designed to prefer the lowest-cost path, not the highest.
C
Distractor review
Gi1/0/23 is blocked because alternate ports are always chosen over root ports.
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
Best answer
Gi1/0/24 advertises a lower path cost to the root bridge.
This is correct because STP prefers the path with the lowest total cost to the root bridge. The exhibit makes that visible: Gi1/0/24 has a cost of 4 and Gi1/0/23 has a cost of 19.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that STP always selects the port with the lowest interface number as the root port. Many candidates mistakenly believe that Gi1/0/24 was chosen simply because it has a lower port number than Gi1/0/23. However, STP first compares root path costs before considering port numbers as tie-breakers. Another trap is thinking that alternate ports are preferred over root ports, which is incorrect because root ports are the primary forwarding paths. Misunderstanding these rules can lead to incorrect conclusions about why a port is blocked or forwarding in STP.
Technical deep dive
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
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: Gi1/0/24 advertises a lower path cost to the root bridge. — Spanning Tree chooses a root port by looking for the best path toward the root bridge. In practical terms, the switch asks, “Which uplink gets me to the root more efficiently?” In the exhibit, 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. At a technical level, STP compares root path cost first. Only if the cost is tied does it move on to tie-breakers such as sender bridge ID and sender port ID. That is why the port number explanation is not the best answer here. The output already gives enough evidence: the lower cost on Gi1/0/24 explains exactly why that port won the root-port election.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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