CCNA Network Infrastructure and Connectivity Practice Question
Which three of the following are characteristics of Layer 2 Ethernet switches that improve network performance? (Choose three.)
⚠ Common exam trap
Cisco often tests the distinction between collision domains and broadcast domains, where candidates mistakenly think switches reduce broadcast domains, but switches only segment collision domains while broadcast domains are controlled by VLANs or routers.
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
✓
They create separate collision domains per port.
Layer 2 Ethernet switches improve network performance by creating separate collision domains per port, eliminating collisions between devices on different ports. They forward frames based on the destination MAC address, enabling efficient hardware-based switching. Cut-through switching reduces latency by starting to forward as soon as the destination MAC address is read. The other options are incorrect: switches do not reduce broadcast domains (broadcasts are forwarded to all ports in the same VLAN unless a router or VLAN segmentation is used); switches operate at Layer 2 using MAC addresses, not IP addresses; and unknown unicast frames are flooded out all ports except the incoming port, not automatically blocked, to ensure connectivity if the destination is unknown.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
They create separate collision domains per port.
Why this is correct
Each switch port terminates the shared medium of a legacy hub, so a collision occurring on one port does not propagate to other ports. Because the switch buffers and forwards frames independently per interface, every port is its own collision domain, enabling full-duplex operation without CSMA/CD collisions. This is a fundamental characteristic that distinguishes switches from hubs.
- ✓
They forward frames based on the destination MAC address.
Why this is correct
A Layer 2 switch examines the destination MAC address in the Ethernet frame header and consults its MAC address table to determine the egress port. If the address is known, the frame is forwarded only out that specific port; this selective forwarding preserves bandwidth and privacy. The switch does not modify the frame payload and does not require Layer 3 information.
- ✗
They reduce the number of broadcast domains.
Why it's wrong here
Switches operate at Layer 2 and do not filter broadcast traffic by default; they flood broadcast frames (destination FF:FF:FF:FF:FF:FF) out all ports except the ingress port, except when VLANs are configured. A switch without VLANs creates a single broadcast domain, not multiple, and it does not segment broadcast domains. Routers or VLANs are required to reduce broadcast-domain size, so this statement is incorrect as a general characteristic.
- ✓
They can perform cut-through switching to reduce latency.
Why this is correct
Cut-through switching begins forwarding a frame as soon as the destination MAC address is read, without waiting for the entire frame to be received. This reduces store-and-forward latency, though it may forward frames with errors or runt frames because the FCS is not checked. Many enterprise switches support cut-through, store-and-forward, or fragment-free modes, making this a valid Layer 2 switch characteristic.
- ✗
They use IP addresses to make forwarding decisions.
Why it's wrong here
Layer 2 switches work with MAC addresses and do not examine IP addresses in the packet header; their forwarding decisions are based solely on Ethernet addressing. IP-address-based forwarding is performed by routers and Layer 3 switches when routing between VLANs, but a pure Layer 2 switch does not use IP addresses. Thus, this is not a characteristic of a Layer 2 Ethernet switch.
- ✗
They automatically block all unknown unicast frames.
Why it's wrong here
When a destination MAC address is unknown, a Layer 2 switch floods the frame out all ports except the ingress port rather than blocking it. This ensures the frame reaches the intended device, and the switch learns the source MAC address of the device when it responds. Blocking unknown unicast frames would break normal Ethernet operations, so this statement is false.
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
cut
In scripting and automation, cut is a command-line utility used to extract specific fields or columns from each line of a file or text stream based on a delimiter or character position.
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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