Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
When spanning tree elects a root bridge, which value is considered first?
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 MAC address only
MAC address matters only if the bridge priorities are equal.
B
Distractor review
Lowest bridge priority only
Priority alone is not the entire comparison value; the full bridge ID is used.
C
Best answer
Lowest bridge ID, which begins with priority
Correct. STP compares the bridge ID, and priority is the leading field in that comparison.
D
Distractor review
Highest interface bandwidth
Interface bandwidth affects path cost, not root bridge election.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that the lowest MAC address alone elects the root bridge. Many candidates overlook that STP first compares the bridge priority field before considering MAC addresses. If priorities differ, the MAC address is irrelevant. Another common mistake is thinking interface bandwidth or path cost influences root bridge election; these factors only affect port roles and forwarding paths after the root is chosen. Misunderstanding this order can lead to incorrect answers and confusion about STP behavior.
Technical deep dive
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 network protocol designed to prevent loops in Ethernet networks by electing a single root bridge. The root bridge acts as the logical center of the network topology, and all path calculations for forwarding frames are made relative to it. The election process is fundamental to STP operation and ensures a loop-free topology by blocking redundant paths. The root bridge election process uses the Bridge ID (BID) as the deciding factor. The BID is a composite value consisting of a configurable bridge priority (default 32768) and the switch’s MAC address. STP compares the BIDs of all switches in the network, first by priority and then by MAC address if priorities are equal. The switch with the lowest BID becomes the root bridge. This hierarchical comparison ensures a deterministic and consistent root bridge selection. A common exam trap is misunderstanding that the MAC address alone or priority alone determines the root bridge. While the MAC address is part of the BID, it is only considered if two switches have the same priority. Also, interface bandwidth or path cost influences port roles and forwarding decisions but does not affect root bridge election. Practically, network engineers can manipulate root bridge election by adjusting bridge priority values to control which switch becomes root, ensuring optimal traffic flow and redundancy.
KKey Concepts to Remember
- STP elects the root bridge by comparing the bridge ID, which consists of a configurable priority and the switch’s MAC address.
- The bridge priority is the first field compared in the bridge ID during root bridge election, with lower values preferred.
- If two switches have the same priority, STP compares their MAC addresses to determine the root bridge, selecting the lowest MAC address.
- Interface bandwidth and path cost influence port roles and forwarding decisions but do not affect root bridge election.
- The root bridge serves as the logical center of the STP topology, and all path calculations are made relative to it.
- Network engineers can influence root bridge election by adjusting the bridge priority to ensure optimal network design.
- STP prevents Layer 2 loops by blocking redundant paths based on root bridge election and port roles.
- The bridge ID comparison process ensures a deterministic and consistent root bridge selection across all switches.
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 elects the root bridge by comparing the bridge ID, which consists of a configurable priority and the switch’s MAC address.
What is the correct answer to this question?
The correct answer is: Lowest bridge ID, which begins with priority — The root bridge is the switch with the lowest bridge ID. The bridge ID is made up of priority and MAC address, so priority is considered first, then MAC address if priorities tie.
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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