Last updated on 2024年9月4日

When should you use VLAN untagging and when should you avoid it?

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VLAN untagging is a process of removing the VLAN ID from a frame when it leaves a switch port. This allows the frame to be compatible with devices that do not support VLANs, such as routers, servers, or end users. However, untagging also has some drawbacks and limitations that you should be aware of before using it. In this article, you will learn when you should use VLAN untagging and when you should avoid it.

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1 What is VLAN tagging?

VLAN tagging is a method of adding a VLAN ID to a frame when it enters a switch port. This allows the switch to identify which VLAN the frame belongs to and forward it accordingly. VLAN tagging is useful for creating logical segments of a network, isolating traffic, and improving security and performance. VLAN tagging can be done in two ways: using the IEEE 802.1Q standard or using proprietary protocols such as Cisco's Inter-Switch Link (ISL).

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Dr Saroj Pradhan

Cloud Network Admin/Consultant/Architect For Cloud & Virtualization,Routing & Switching Azure Cloud, SDN ACI,VXLAN,VPN,VDC For DC. Cisco ISE ,LB F5 ,Azure LB,APP GW ,Azure Firewall, Paloalto, Fortinet,ASA
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VLAN tagging----When a device sends data onto the network, it includes a field called a tag that identifies which VLAN the data should be sent to. The switch looks at this tag and then forwards the data to all devices in that VLAN. One advantage of using VLANs is that they can help reduce broadcast traffic on the network.

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2 What is VLAN untagging?

VLAN untagging is the opposite of VLAN tagging. It is a process of removing the VLAN ID from a frame when it leaves a switch port. This allows the frame to be compatible with devices that do not support VLANs, such as routers, servers, or end users. VLAN untagging can be done in two ways: using the native VLAN or using the access mode.

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Dr Saroj Pradhan

Cloud Network Admin/Consultant/Architect For Cloud & Virtualization,Routing & Switching Azure Cloud, SDN ACI,VXLAN,VPN,VDC For DC. Cisco ISE ,LB F5 ,Azure LB,APP GW ,Azure Firewall, Paloalto, Fortinet,ASA
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VLAN untagging-----VLAN untagging is the opposite of VLAN tagging. A switch-port can be configured as access or trunk. In access mode, incoming frames are tagged with the vlan-id that the port belongs to. An access port expects untagged frames.

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3 When to use the native VLAN?

The native VLAN is a special VLAN that is assigned to a trunk port. A trunk port is a switch port that can carry multiple VLANs using VLAN tagging. The native VLAN is the VLAN that is used for untagged frames that enter or leave a trunk port. The native VLAN is useful for communicating with devices that do not support VLANs, such as legacy equipment or management stations. However, the native VLAN should be consistent across all trunk ports in a network, otherwise it can cause VLAN mismatch errors or security risks.

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Dr Saroj Pradhan

Cloud Network Admin/Consultant/Architect For Cloud & Virtualization,Routing & Switching Azure Cloud, SDN ACI,VXLAN,VPN,VDC For DC. Cisco ISE ,LB F5 ,Azure LB,APP GW ,Azure Firewall, Paloalto, Fortinet,ASA
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Native VLAN is generally a dot 1Q conception elaborated for backward compatibility with devices that don’t endorse VLANs. Frames that are derived from the native VLAN are not tagged when transferred out on the trunk links. Frames entered untagged on the trunk links are set to the native VLAN. VLANs can be as multitudinous as there are.1 Q trunks on the switch. The Native VLAN is one conception defined in the 802.1 q grade that was created for backward compatibility with the old device that does not endorse VLANs. The concept at the back of the local VLAN is that a few network traffic still flows over the entire switched network even if the trunk fails.

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4 When to use the access mode?

The access mode is a switch port configuration that allows only one VLAN to pass through. The access mode is used for connecting devices that do not support VLANs, such as end users or servers. The access mode untags the frames that leave the switch port and tags the frames that enter the switch port with the assigned VLAN. The access mode is useful for simplifying the network design and reducing the overhead of VLAN tagging. However, the access mode limits the flexibility and scalability of the network, as each device can only belong to one VLAN.

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5 When to avoid VLAN untagging?

Using VLAN untagging can be risky, as it can expose your network to security threats such as VLAN hopping or spoofing attacks. Additionally, it can reduce the performance and reliability of your network, potentially causing broadcast storms, collisions, or congestion. Moreover, it can make troubleshooting and monitoring more difficult, which can affect the quality of service and the service level agreements of your network.

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6 How to choose the best option?

When determining the best option for using VLAN untagging, you should consider the type and number of devices that you want to connect to your network, the level of security and performance that you want to achieve, and the complexity and scalability of your network design. If you have devices that do not support VLANs, VLAN untagging may be necessary. On the other hand, if your devices support VLANs, you may be able to take advantage of its benefits through VLAN tagging. Additionally, if you want to increase the security and performance of your network, VLAN untagging should be avoided. However, if your network is low-risk and low-demand, VLAN untagging can be used with proper precautions and configurations. Furthermore, if you want to simplify and reduce the complexity of your network design, VLAN untagging is a good option. Meanwhile, VLAN tagging can create more logical segments and subnets for increased flexibility and scalability.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Cmdr (Dr.?) Reji Kurien Thomas , FRSA, MLE?

I Empower Sectors as a Global Tech & Business Transformation Leader| Stephen Hawking Award| Harvard Leader | UK House of Lord's Awardee | Fellow Royal Society | CyberSec I 200x LinkedIn Top Voice | CCISO CISM
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Some legacy devices do not support VLAN tagging, so untagging may be necessary to ensure compatibility. In a manufacturing facility I managed, we had to support older networked equipment that could not recognise tagged VLAN frames. By configuring access ports with untagged VLANs specific to those devices, we were able to integrate them into the network without upgrading or replacing hardware. Using VLAN untagging for legacy devices enables network administrators to extend VLAN functionality to devices that would otherwise not be compatible, balancing modern & older infrastructure.

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Dr Saroj Pradhan

Cloud Network Admin/Consultant/Architect For Cloud & Virtualization,Routing & Switching Azure Cloud, SDN ACI,VXLAN,VPN,VDC For DC. Cisco ISE ,LB F5 ,Azure LB,APP GW ,Azure Firewall, Paloalto, Fortinet,ASA
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What is VTP (VLAN Trunking Protocol)? VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used by Cisco switches to exchange VLAN information. With VTP, you can synchronize VLAN information (such as VLAN ID or VLAN name) with switches inside the same VTP domain. A VTP domain is a set of trunked switches with the matching VTP settings (the domain name, password and VTP version). All switches inside the same VTP domain share their VLAN information with each other.

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When should you use VLAN untagging and when should you avoid it?

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1 What is VLAN tagging?

2 What is VLAN untagging?

3 When to use the native VLAN?

4 When to use the access mode?

5 When to avoid VLAN untagging?

6 How to choose the best option?

7 Here’s what else to consider

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