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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   P2p

Based on the exhibit, why did interface Gi1/0/24 become the root port?

Question 1hardmultiple choice
Full question →

Based on the exhibit, why did interface Gi1/0/24 become the root port?

Full question →

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

Best answer

Because Gi1/0/24 has the lower path cost to the root bridge.

This is correct because STP selects the lowest-cost path as the root port.

B

Distractor review

Because Gi1/0/24 has the higher port number.

This is wrong because STP does not prefer higher port numbers.

C

Distractor review

Because alternate ports always become the root port first.

This is wrong because alternate ports are backup paths, not the preferred forwarding path.

D

Distractor review

Because root ports are always chosen by the highest interface priority.

This is wrong because path cost is the deciding factor shown here.

Common exam trap

Common exam trap: answer the scenario, not the keyword

A frequent exam trap is selecting an answer based on port number or interface priority rather than path cost. Some candidates mistakenly believe that the highest port number or interface priority determines the root port, but STP prioritizes the lowest path cost to the root bridge. Alternate ports are backup ports and never become root ports unless the root port fails. Misinterpreting these roles leads to incorrect conclusions about STP behavior. Always remember that STP’s root port selection is fundamentally about finding the lowest-cost path to the root bridge, not about port numbering or priority.

Technical deep dive

How to think about this question

The Spanning Tree Protocol (STP) is designed to prevent Layer 2 switching loops by creating a loop-free logical topology. Each switch in the STP domain elects a root bridge, which is the logical center of the network. Non-root switches select a single root port, which is the port with the lowest path cost to reach the root bridge. The path cost is cumulative and depends on the speed of the links along the path. Lower path costs indicate more efficient routes to the root. STP’s root port selection process begins by comparing the total path cost to the root bridge for each port. The port with the lowest path cost becomes the root port. If there is a tie in path cost, STP uses the sender bridge ID and then the port ID as tie-breakers. Alternate ports are designated as backup ports and do not forward traffic toward the root unless the root port fails. Interface priority or port number alone does not determine root port selection. A common exam trap is assuming that the root port is chosen based on the highest port number or interface priority, which is incorrect. The root port is always the port with the lowest path cost to the root bridge. Practically, this means that network engineers must carefully analyze STP output to identify the root port by examining path costs rather than relying on port numbers or priorities. This understanding helps in troubleshooting and optimizing STP-based network topologies.

KKey Concepts to Remember

  • Spanning Tree Protocol (STP) selects a single root port on each non-root switch as the port with the lowest path cost to the root bridge.
  • STP calculates path cost based on the cumulative cost of all links from the switch to the root bridge, preferring lower total cost paths.
  • If multiple ports have the same path cost, STP uses tie-breakers such as the lowest sender bridge ID and then the lowest port ID to select the root port.
  • Alternate ports in STP serve as backup paths and do not become root ports unless the current root port fails or is removed.
  • STP does not select root ports based on higher port numbers or interface priorities; path cost is the primary selection criterion.
  • The root port is the single port on a non-root switch that forwards frames toward the root bridge, ensuring a loop-free topology.
  • STP blocks redundant paths by placing non-root ports in a blocking state to prevent loops while maintaining network redundancy.
  • Understanding STP root port selection is critical for interpreting STP output and troubleshooting network topology issues in Cisco environments.

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?

Spanning Tree Protocol (STP) selects a single root port on each non-root switch as the port with the lowest path cost to the root bridge.

What is the correct answer to this question?

The correct answer is: Because Gi1/0/24 has the lower path cost to the root bridge. — Gi1/0/24 became the root port because it has the lower path cost to the root bridge. In practical terms, spanning tree prefers the best path toward the root, and lower cost is better. Only if the path cost is tied does the protocol move to later tie-breakers such as bridge ID or port ID. This is a pure STP output-reading question, and those are close to real exam style because they reward careful interpretation rather than memorized syntax.

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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