For a CCNA spanning tree root bridge question, compare bridge IDs before looking at link speed or the switch’s position in the drawing. The elected root has the lowest bridge ID within the spanning-tree instance. A diagram that places one switch in the middle does not automatically make it the root.
An original three-switch exercise
Assume all three switches participate in the same VLAN and spanning-tree instance. Their configured bridge priorities and MAC addresses are:
- Switch A: priority 32768, MAC 0000.0000.0010.
- Switch B: priority 24576, MAC 0000.0000.0030.
- Switch C: priority 24576, MAC 0000.0000.0020.
Which becomes the root? Switch C.
First compare priority. B and C have lower values than A, so A loses even though its MAC address is numerically smallest. Then compare B and C, which tie on priority. C’s lower MAC address breaks the tie. The VLAN-related system ID extension is the same in this example and does not change the ordering.
Change one fact
Give Switch A a configured priority of 16384 while leaving everything else unchanged. A now wins. There is no need to compare MAC addresses because the priority comparison already decides the election.
This is the kind of variation worth making after a practice question. If you only remember that “C won,” the next diagram may expose the gap. Try the CCNA practice questions and write down which comparison eliminated each alternative.
Root bridge is not root port
Root bridge selection identifies a switch. Root port selection identifies a non-root switch’s best port toward the root, using path information and tie-breakers. Combining these into one calculation is a common source of confusion.
A faster link may affect path cost and therefore a port decision. It does not replace the bridge-ID comparison used to elect the root. Cisco’s spanning-tree guide describes these related but separate parts of STP operation.
A short lab check
In an isolated lab, inspect the root ID and local bridge ID in spanning-tree output for one VLAN. Identify whether the local switch is the root, then compare what happens when you intentionally change a bridge priority. Keep configuration experiments out of a production network.
Use the CCNA network access study guide to connect the election with loop prevention and port roles. Your review note should explain why the chosen root wins and why an attractive distractor—such as the fastest or most central switch—does not answer this particular question.