CCNA Network Infrastructure and Connectivity Practice Question
Which three statements about the Spanning Tree Protocol (STP) are true? (Choose three.)
⚠ Common exam trap
Cisco often tests the fact that STP does not use timers to elect the root bridge (it uses bridge ID comparison) and that blocking state is the mechanism for loop prevention, not disabling the port entirely or relying on TCN BPDUs alone.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
STP uses Bridge Protocol Data Units (BPDUs) to exchange topology information.
All three statements are correct because STP relies on Bridge Protocol Data Units (BPDUs) to share topology information between switches, elects a root bridge by comparing bridge IDs (a combination of priority and MAC address, with the lowest value winning), and prevents loops by placing redundant ports into a blocking state (discarding state in Rapid PVST+). These are fundamental behaviors of the 802.1D Spanning Tree Protocol.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
STP uses Bridge Protocol Data Units (BPDUs) to exchange topology information.
Why this is correct
STP relies on BPDUs as its control plane messaging mechanism. Each switch sends BPDUs containing its bridge ID, root path cost, and port identifiers, allowing all switches to share a consistent view of the Layer 2 topology. These frames are how switches elect a root bridge and determine which ports should be forwarding; BPDUs are continuously exchanged even after convergence to detect link failures.
- ✓
STP elects a root bridge based on the lowest bridge ID.
Why this is correct
The root bridge is selected by comparing the bridge ID, which is an 8-byte value made up of a 2-byte priority field (default 32768) and a 6-byte MAC address. Switches parse incoming BPDUs and compare the sender's bridge ID against their own and against other received BPDUs; the switch with the numerically lowest bridge ID wins the election. This deterministic selection ensures a single root for the spanning tree instance.
- ✓
STP places redundant ports in blocking state to prevent loops.
Why this is correct
To prevent broadcast storms and MAC address table instability, STP places redundant ports in the blocking state. While a root port and designated ports forward traffic, any port that is neither becomes a non-designated port and is blocked; it still listens to BPDUs but does not send or receive data frames. This ensures only one active Layer 2 path exists between any two switches, breaking potential loops.
- ✗
STP always uses the highest port cost to select the root port.
Why it's wrong here
STP does not use the highest port cost to select the root port; it uses the lowest root path cost. The root port on each non-root switch is the port offering the shortest path back to the root bridge, measured by cumulative path cost. Higher costs represent slower or less desirable links, so selecting the highest cost would be counterproductive and is the opposite of the actual selection criterion.
- ✗
STP converges instantly after a topology change.
Why it's wrong here
STP does not converge instantly after a topology change due to its timer-based state transitions. A port moves through blocking, listening, and learning states—each requiring a forwarding delay of 15 seconds—before reaching forwarding, totaling roughly 30 seconds. This delay is intentional, giving the network time to propagate updated BPDUs and ensure no temporary loops form before the new topology is stable.
- ✗
STP is used to increase the number of broadcast domains.
Why it's wrong here
STP is not a mechanism for increasing the number of broadcast domains; broadcast domains are determined by Layer 3 boundaries, such as VLANs or subnets. STP operates strictly within a single broadcast domain to manage redundant paths and eliminate Layer 2 loops. Increasing broadcast domains is accomplished by segmenting the network with routers or VLANs, not by running STP.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
Bridge Protocol Data Unit
A Bridge Protocol Data Unit (BPDU) is a special message that network switches exchange to detect and prevent loops in an Ethernet network.
Key term
MAC
MAC (Media Access Control) is a unique hardware identifier assigned to network interfaces for communication on a local network segment.
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