Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SW2 uplink A root path cost = 4 SW2 uplink B root path cost = 19 Both links receive superior BPDUs
Exhibit: SW2 receives superior BPDUs on both uplinks. One uplink becomes the root port and the other becomes alternate. Which factor is considered first when SW2 chooses the root port?
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
Lowest local interface MAC address
MAC address is not the first deciding factor for root port selection.
B
Best answer
Lowest root path cost
That is the first major comparison.
C
Distractor review
Highest duplex setting
Duplex is not a root port tie-breaker.
D
Distractor review
Lowest configured VLAN number
VLAN number is not used to choose a root port.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that the root port selection depends on the local interface MAC address or duplex settings. Some candidates mistakenly believe that the switch picks the root port based on the lowest MAC address of its own interfaces or prefers a port with higher duplex. However, STP root port selection is based on the lowest root path cost advertised in received BPDUs, not local interface attributes. This misunderstanding can lead to incorrect answers, especially when multiple uplinks receive superior BPDUs. Remember, local MAC addresses and duplex settings do not influence root port choice in STP.
Technical deep dive
How to think about this question
Spanning Tree Protocol (STP) is designed to prevent Layer 2 loops in Ethernet networks 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 determine their best path to the root bridge by selecting a single root port. This root port is the port with the lowest cumulative cost to reach the root bridge, known as the root path cost. The root path cost is calculated by adding the cost values of each link along the path to the root bridge, where lower cost links are preferred. When a switch receives superior BPDUs on multiple ports, it compares the root path cost advertised in those BPDUs to select the root port. The port with the lowest root path cost becomes the root port. If two or more ports have the same root path cost, STP uses tie-breakers in a specific order: first, the sender bridge ID (the MAC address and priority of the switch sending the BPDU) is compared, and if still tied, the sender port ID is compared. These tie-breakers ensure a deterministic and loop-free topology. Local interface MAC addresses, duplex settings, or VLAN numbers are not factors in root port selection. A common exam trap is to confuse the root port selection criteria with local interface characteristics such as MAC address or duplex settings. Candidates might incorrectly assume the lowest MAC address or highest duplex port is chosen first, but STP strictly follows root path cost and bridge/port IDs. Practically, the alternate port is a blocked port that provides redundancy; it becomes the root port only if the current root port fails. Understanding this selection process is critical for troubleshooting STP issues and ensuring network stability in Cisco environments.
KKey Concepts to Remember
- STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
- The root path cost is the cumulative cost of all links from the switch to the root bridge, calculated using standard STP cost values.
- If multiple ports receive superior BPDUs with the same root path cost, STP uses the sender bridge ID as the first tie-breaker.
- If the sender bridge ID is also equal, STP then compares the sender port ID to break ties and select the root port.
- STP does not consider local interface MAC addresses or duplex settings when selecting the root port.
- VLAN numbers do not influence root port selection because STP operates per VLAN but chooses root ports based on path cost and bridge IDs.
- An alternate port is a backup port that is blocked to prevent loops but can become the root port if the current root port fails.
- STP ensures a loop-free topology by selecting one root port per switch and blocking redundant paths using alternate ports.
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?
STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
What is the correct answer to this question?
The correct answer is: Lowest root path cost — STP chooses the best path to the root bridge based first on the lowest root path cost. If the cost ties, it then checks the sender bridge ID and sender port ID as tie-breakers.
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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