Cisco VLAN Configuration and Troubleshooting Lab

Cisco VLAN configuration becomes easier when you practice it as a complete lab: create VLANs, assign access ports, configure an 802.1Q trunk, verify forwarding, and troubleshoot common mistakes. This guide complements our conceptual VLAN introduction by focusing on hands-on switch configuration.

VLAN Lab Topology

Use two Cisco switches connected through a trunk. Connect one PC in VLAN 10 and one PC in VLAN 20 on each switch. Devices in the same VLAN should communicate across the trunk, while communication between VLANs requires a router or multilayer switch.

Step 1: Create and Name VLANs

enable
configure terminal
vlan 10
 name STAFF
vlan 20
 name SALES
vlan 99
 name MANAGEMENT
end

Create the required VLANs on every switch that must forward their traffic. A VLAN missing from one switch can interrupt connectivity even when the trunk is operational.

Step 2: Configure Access Ports

configure terminal
interface range gigabitEthernet 0/1 - 2
 switchport mode access
 switchport access vlan 10
 spanning-tree portfast
exit
interface range gigabitEthernet 0/3 - 4
 switchport mode access
 switchport access vlan 20
 spanning-tree portfast
end

Use PortFast only on appropriate edge ports connected to endpoints. Do not apply it carelessly to links where switches may appear.

Step 3: Configure the Trunk

configure terminal
interface gigabitEthernet 0/24
 description TRUNK_TO_SW2
 switchport mode trunk
 switchport trunk native vlan 99
 switchport trunk allowed vlan 10,20,99
 no shutdown
end

Configure the other end with matching native and allowed VLAN settings. Native-VLAN mismatches can produce warnings and unexpected forwarding behavior.

Step 4: Verify the VLANs

• show vlan brief — checks VLAN existence and access-port membership.
• show interfaces trunk — checks trunk operation, native VLAN, and allowed VLANs.
• show interfaces switchport — shows administrative and operational switchport settings.
• show mac address-table dynamic — confirms MAC learning by VLAN and interface.
• show spanning-tree vlan 10 — checks forwarding state and the root path.

Step 5: Test Connectivity

Assign IP addresses from the same subnet to endpoints in the same VLAN. Test local connectivity first, then test across the trunk. If same-VLAN communication succeeds but different-VLAN communication fails, confirm that inter-VLAN routing is configured.

Common VLAN Troubleshooting Scenarios

Access Port in the Wrong VLAN

Compare the intended design with show vlan brief and show interfaces switchport. Correct the access VLAN and verify the endpoint address.

VLAN Missing from the Trunk

Use show interfaces trunk. Confirm the VLAN exists locally and appears in the allowed and forwarding lists.

Trunk Not Forming

Check interface status and the configured switchport mode on both ends. Confirm the connection uses the intended interfaces and no physical error is present.

Native VLAN Mismatch

Configure the same native VLAN on both trunk endpoints. Review neighbor messages and trunk output after correcting the mismatch.

No Inter-VLAN Communication

Verify switched virtual interfaces or router subinterfaces, gateway addresses, Layer 3 routing, access lists, and interface status.

Save the Configuration

copy running-config startup-config

Save only after verifying that the lab operates as expected.

Frequently Asked Questions

Do VLANs need to exist on both switches?

Yes, when both switches must carry or use those VLANs and no supported VLAN-distribution mechanism creates them automatically.

Can two different VLANs use the same IP subnet?

Normal design maps each VLAN to a separate subnet to maintain clear Layer 2 and Layer 3 boundaries.

Where can I practice more switching questions?

Review our CCNA interview questions and answers. For focused preparation, see the Cisco 200-301 Premium PDF with the latest available questions and answers.

CiscoForAll provides independent study and practice material. Always use official Cisco documentation as a primary preparation source.