In a switched Ethernet network, which switching technology is specifically used to reduce the size of a broadcast domain by logically segmenting a single physical network into multiple separate logical networks?

Introduction / Context:
In Ethernet networks, a broadcast domain is the set of devices that receive broadcast frames from one another. Large broadcast domains can lead to excessive broadcast traffic and reduced performance. A key design goal in modern networks is to reduce the size of broadcast domains while still using shared physical infrastructure. Virtual LANs, or VLANs, provide a way to achieve this by logically segmenting the network. This question asks you to identify which switching technology directly reduces the size of a broadcast domain.

Given Data / Assumptions:

• The network uses Ethernet switches for layer 2 connectivity.
• There is a need to reduce the size of broadcast domains for performance and security reasons.
• Multiple technologies are available, including trunking, VLANs, STP, and link aggregation.
• The question focuses on which technology reduces the broadcast domain size itself, not just improves redundancy or capacity.

Concept / Approach:
VLANs logically divide a single physical switch into multiple virtual switches, each with its own separate broadcast domain. Hosts in one VLAN do not receive broadcast frames from hosts in another VLAN unless a router or layer 3 device forwards traffic between them. Trunking protocols like ISL and IEEE 802.1Q carry multiple VLANs over a single link but do not, by themselves, create segmentation; they simply transport VLAN information. Spanning Tree Protocol prevents loops but does not change broadcast domain size. EtherChannel aggregates links for more bandwidth and redundancy but also leaves the broadcast domain unchanged. Therefore, VLANs are the technology that directly reduces broadcast domain size.

Step-by-Step Solution:
1. Recall that a broadcast domain is the set of devices that receive each other's broadcast frames. 2. Understand that VLANs create separate logical layer 2 segments, each acting as an independent broadcast domain. 3. Recognize that ISL and 802.1Q are tagging methods that allow VLANs to traverse trunk links, but the VLAN construct itself is what defines domain boundaries. 4. Note that STP only blocks ports to prevent loops; it does not change which devices share broadcasts. 5. Note that EtherChannel combines physical links into a logical link for bandwidth and redundancy, but devices on either side still share the same broadcast domain. 6. Conclude that Virtual LANs are the switching technology that reduces broadcast domain size.

Verification / Alternative check:
In practice, when you configure multiple VLANs on a switch and assign different ports to different VLANs, a broadcast sent on VLAN 10 is not delivered to ports in VLAN 20. This is easy to confirm with packet sniffer tools. Only traffic that goes through a router or a layer 3 switch can move between VLANs, which proves that VLANs define broadcast domain boundaries. Trunks are simply used so that multiple VLANs can share an uplink while remaining logically separated.

Why Other Options Are Wrong:
Option a: ISL is a Cisco proprietary trunking protocol that encapsulates VLAN information in frames, but it does not inherently split broadcast domains; VLANs do that. Option b: IEEE 802.1Q is a standard trunking method that inserts VLAN tags inside Ethernet frames, again used to carry VLANs over links rather than define broadcast domain size by itself. Option d: STP prevents switching loops and resulting broadcast storms, but the entire switched domain still remains one broadcast domain unless VLANs are configured. Option e: EtherChannel aggregates multiple physical links into one logical link for redundancy and higher bandwidth; broadcast domain size remains unchanged.

Common Pitfalls:
Learners often confuse trunking with VLANs and assume that enabling 802.1Q trunking alone creates isolation. In reality, trunks only carry existing VLANs across links. The real segmentation comes from VLAN configuration on switch ports. Another pitfall is thinking that STP or link aggregation technologies somehow reduce broadcast traffic by splitting domains. They may improve stability or capacity but do not change broadcast domain boundaries. Always remember that VLAN equals broadcast domain at layer 2, which simplifies reasoning about exam questions and design decisions.

Final Answer:
The switching technology that reduces the size of a broadcast domain is Virtual LANs (VLANs).