show vlan

5.4 Troubleshooting Tools

Basic Network Device Command

📘CompTIA Network+ (N10-009)

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What is VLAN?

• VLAN stands for Virtual Local Area Network.
• A VLAN is a logical network created within a physical network switch.
• It allows network admins to group devices together even if they are not physically connected to the same switch port.
• VLANs help in:
  • Segmentation: Separating traffic for security or performance.
  • Reducing broadcast domains: Only devices in the same VLAN see each other’s broadcast traffic.
  • Simplifying network management: Easier to assign users or devices to networks logically.

Example in IT terms: A company can have VLAN 10 for the finance department and VLAN 20 for the HR department. Even if both sets of computers are connected to the same switch, their traffic is logically separated.

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What the show vlan Command Does

The show vlan command is used on network switches (like Cisco switches) to display all VLANs configured on the device.

When you run this command, you can see:

1. VLAN ID – The unique number assigned to each VLAN.
   • Example: 1, 10, 20
   • VLAN 1 is usually the default VLAN on most switches.
2. VLAN Name – The name assigned to the VLAN for easier identification.
   • Example: VLAN10 may be named Finance.
3. Status – Shows if the VLAN is active or suspended.
   • active means the VLAN is working and can be used.
   • suspended means it exists in the config but is not operational.
4. Ports Assigned – Lists the switch ports that belong to each VLAN.
   • Example: Ports Fa0/1 and Fa0/2 might belong to VLAN 10.

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Typical Output of show vlan

Here’s an example of what you might see when running show vlan:

VLAN Name                             Status    Ports
---- -------------------------------- --------- ----------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3
10   Finance                          active    Fa0/4, Fa0/5
20   HR                               active    Fa0/6, Fa0/7

Explanation:

• VLAN 1 is the default VLAN that all ports belong to initially.
• VLAN 10 is created for the finance team and has ports 4 and 5.
• VLAN 20 is created for HR with ports 6 and 7.

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Key Points to Remember for the Exam

1. show vlan is a read-only command – It only shows configuration; it doesn’t change anything.
2. Used to troubleshoot VLAN issues – If devices can’t communicate, check if they are on the same VLAN using show vlan.
3. Default VLAN is VLAN 1 – Important for exam questions; some exams may ask what VLAN a port belongs to by default.
4. VLAN status matters – Only active VLANs can forward traffic.
5. Port assignment shows how devices are connected logically – Helps in network segmentation and security.

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How show vlan is Used in a Real IT Environment

• Troubleshooting network connectivity:
  • If a user can’t access the finance server, a network admin can check if their port is in VLAN 10.
• Planning network changes:
  • Admins can see which ports are assigned to each VLAN before moving devices or creating new VLANs.
• Security checks:
  • Ensures sensitive departments are on separate VLANs to avoid unauthorized access.

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Quick Exam Tip

• Remember: show vlan = check VLAN configuration
• Related commands that might also appear in questions:
  • show vlan brief – Shows VLANs in a concise format.
  • show running-config – You can see VLANs configured in the switch config.

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✅ Summary
The show vlan command is essential for verifying VLAN setups on a switch. It lists VLAN IDs, names, status, and assigned ports, which helps in network troubleshooting, management, and security. For CompTIA Network+, you should understand what it shows, how to read it, and why it’s useful in an IT environment.