Read local delivery and subnetting.
VLAN broadcast domain separation
A virtual LAN (VLAN) divides a switch into separate Layer 2 broadcast domains. Think of departments using separate meeting rooms even though they share the same building. Ordinary access ports place connected hosts in one data VLAN. A VLAN number is not an IP subnet, although networks commonly assign one subnet per VLAN.
A trunk can carry multiple VLANs between devices. IEEE 802.1Q adds a VLAN tag to identify traffic. Native VLAN handling matters for untagged traffic; configure both ends consistently. A trunk’s allowed list can exclude a VLAN even when that VLAN exists on both switches.
VLAN and trunk connectivity checks
Two PCs in different VLANs do not gain a path just because they have addresses in the same /24. Communication between VLANs needs Layer 3 routing and suitable IP subnets. A router subinterface can act as a gateway for each VLAN in a router-on-a-stick design. A multilayer switch can instead use switched virtual interfaces (SVIs).
A voice VLAN lets a supported phone use its voice network while a connected PC uses a data VLAN. VLAN 1 is the default VLAN on many Catalyst switches, but “default”, “native” and “management” describe different roles. VLAN separation supports organisation and policy; it is not encryption.
Verification commands
Use these viewing commands to inspect the idea in this lesson. Start in the environment shown above each example.
show vlan brief
show interfaces trunkCommand reference · enter in the stated environment; output varies by device.
Practical exercise
Predict whether two PCs in VLAN 10 can ping across two switches if the trunk allows only VLAN 20.
Show the answer and explanation
They cannot use that trunk for VLAN 10. Check VLAN existence, access-port membership, trunk state and allowed VLANs before changing host addresses.
QUICK KNOWLEDGE CHECK
Check your understanding
Related labs and exercises
Sources & content checks
Technical references checked on 25 October 2026. These are original teaching explanations, not official Cisco course material. See how content and lab validation are described.
Your progress
Mark this complete when you can explain the example and do the exercise.