Why can’t devices in different VLANs communicate on a Cisco switch? Learn how a Layer 3 device enables inter-VLAN routing and what’s required for cross-VLAN communication—essential for Cisco Certified Field Technician (CCT) exam success.
Table of Contents
Question
A switch is configured with VLANs, but devices in different VLANs cannot communicate. What is the most likely cause?
A. VLANs are not assigned to interfaces
B. The switch is missing a default gateway
C. A Layer 3 device is required for inter-VLAN routing
D. The spanning-tree protocol is blocking the VLAN
E. The switch does not support VLANs
Answer
C. A Layer 3 device is required for inter-VLAN routing
Explanation
VLANs operate at Layer 2, and a Layer 3 device (such as a router or Layer 3 switch) is needed for communication between VLANs.
The most likely reason devices in different VLANs cannot communicate is that a Layer 3 device is required for inter-VLAN routing.
- VLANs (Virtual Local Area Networks) segment network traffic at Layer 2 (Data Link Layer), isolating broadcast domains. Devices within the same VLAN can communicate directly through the switch.
- Communication between devices in different VLANs requires routing, which operates at Layer 3 (Network Layer). Standard Layer 2 switches cannot route traffic between VLANs.
- To enable inter-VLAN communication, a router (using “router-on-a-stick” configuration) or a Layer 3 switch (with switched virtual interfaces, SVIs) must be configured to perform routing between VLANs.
- Without a Layer 3 device configured for inter-VLAN routing, traffic between VLANs is blocked by design, maintaining network segmentation and security.
VLANs isolate devices at Layer 2; a Layer 3 device is necessary to route traffic between VLANs and enable communication across different network segments.
Cisco Certified Field Technician (CCT) certification exam practice question and answer (Q&A) dump with detail explanation and reference available free, helpful to pass the Cisco Certified Field Technician (CCT) exam and earn Cisco Certified Field Technician (CCT) certification.