Understanding VLAN Tagging

Oct 13, 2025 by wbrum.org 103 views

In computer networking, VLANs (Virtual Local Area Networks) are used to segment a network into smaller, isolated groups of devices. This helps to improve network security, reduce congestion, and increase overall network efficiency. When working with VLANs, it's essential to understand how to configure trunk ports, which are used to connect multiple VLANs together. In this article, we'll explore the command used to assign all untagged packets on a trunk to VLAN 999.

What is a Trunk Port?

A trunk port is a network port that carries traffic from multiple VLANs. It's used to connect multiple switches or routers together, allowing them to communicate with each other. Trunk ports are typically used in network core or distribution layers, where multiple VLANs need to be connected.

Understanding VLAN Tagging

VLAN tagging, also known as 802.1Q tagging, is a method used to identify the VLAN that a packet belongs to. When a packet is sent over a trunk port, it's tagged with the VLAN ID, which allows the receiving switch to determine the VLAN that the packet belongs to. There are two types of VLAN tagging:

Tagged packets: These are packets that have a VLAN ID attached to them.
Untagged packets: These are packets that do not have a VLAN ID attached to them.

Configuring VLANs on Trunk Ports

When configuring VLANs on trunk ports, you need to specify which VLANs are allowed to be sent over the trunk port. This is done using the switchport trunk allowed vlan command. However, in this case, we're looking for the command that assigns all untagged packets on a trunk to VLAN 999.

The Correct Command

The correct command to assign all untagged packets on a trunk to VLAN 999 is:

C. switchport trunk native vlan 999

This command sets the native VLAN for the trunk port to VLAN 999. The native VLAN is the VLAN that is used for untagged packets. When a packet is sent over a trunk port without a VLAN ID, it's assumed to be part of the native VLAN.

Why is this the Correct Command?

The switchport trunk native vlan command is used to set the native VLAN for the trunk port. When a packet is sent over a trunk port without a VLAN ID, it's assumed to be part of the native VLAN. By setting the native VLAN to VLAN 999, all untagged packets on the trunk port will be assigned to VLAN 999.

Other Options

The other options listed are not correct for assigning all untagged packets on a trunk to VLAN 999.

A. switchport trunk pruning vlan add 999: This command is used to add a VLAN to the pruning list, which is used to remove VLANs from the trunk port that are not being used.
B. switchport trunk allowed vlan 999: This command is used to add a VLAN to the allowed VLAN list, which specifies which VLANs are allowed to be sent over the trunk port.
D. switchport trunk allowed vlan add 999: This command is similar to option B, but it's used to add a VLAN to the allowed VLAN list.

Conclusion

In conclusion, the correct command to assign all untagged packets on a trunk to VLAN 999 is switchport trunk native vlan 999. This command sets the native VLAN for the trunk port to VLAN 999, which means that all untagged packets on the trunk port will be assigned to VLAN 999.

Common Use Cases

This command is commonly used in network configurations where multiple VLANs need to be connected together using a trunk port. It's essential to understand how to configure VLANs on trunk ports to ensure that network traffic is properly segmented and secured.

Best Practices

When configuring VLANs on trunk ports, it's essential to follow best practices to ensure that network traffic is properly segmented and secured. Some best practices include:

Use VLANs to segment the network: VLANs should be used to segment the network into smaller, isolated groups of devices.
Use trunk ports to connect VLANs: Trunk ports should be used to connect multiple VLANs together.
Configure VLANs on trunk ports carefully: VLANs should be configured on trunk ports carefully to ensure that network traffic is properly segmented and secured.

Troubleshooting Tips

When troubleshooting VLAN configurations on trunk ports, it's essential to follow these tips:

Verify VLAN configuration: Verify that the VLAN configuration is correct and that the native VLAN is set to the correct VLAN.
Check for VLAN tagging: Check that VLAN tagging is enabled on the trunk port.
Verify trunk port configuration: Verify that the trunk port is configured correctly and that the allowed VLAN list is set to the correct VLANs.

By following these best practices and troubleshooting tips, you can ensure that your VLAN configurations on trunk ports are properly configured and that network traffic is properly segmented and secured.