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How virtual VLANs work on VMware ESXi hosts

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Devices within a VLAN (Virtual Local Area Network) can directly communicate with each other at the data link level, even if they are physically connected to different network switches. On the other hand, devices in different VLANs cannot interact with each other at the data link layer, even if they are connected to the same switch. Instead, communication between these devices is only possible at the network and higher layers. Virtual VLANs allow for the grouping of switch ports, creating isolated groups that include virtual machines connected to them. This enables the network to be divided based on organizational or physical principles. The organizational approach is preferable because it eliminates the need for network reconfiguration when a computer is moved to a different physical location. To establish a virtual network VLAN, the VLAN ID must be specified when creating a port group on the virtual switch. Alternatively, physical switch tags or frame tagging can be used at the guest level. By isolating traffic at the virtual switch port level, processor resources and bandwidth on guest system network adapters are conserved.

To implement virtual VLANs in a VMware virtual network, an 802.1Q tag is added to the Ethernet frame.

Below are three options for adding tags to frames to organize virtual VLANs.

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1. Tagging mode at the virtual switch level (Virtual switch tagging, VST).

This is the most commonly used configuration.In this mode, a group of ports is created on the virtual switch for each of the VLANs, after which the virtual network adapter of the virtual machine is bound to this group of ports.Within a group of ports on a virtual switch, outgoing frames are tagged and incoming frames are removed.At the same time, the virtual switch ensures that each packet falls only into its own VLAN.In this case, there is no need to set a virtual VLAN on the physical switch for the port to which the ESX server connects.Using this mode requires trunking mode for the physical switch port, since groups of virtual switch ports may be in different VLANs.

In order to set the tagging mode for the virtual switch, you will need to specify the VLAN ID value within the properties of the virtual switch port group. This value should fall within the range of 1 to 4094, including both numbers.

2. Tagging mode at the guest system level (Virtual machine guest tagging, VGT)

This method necessitates the installation of the 802.1Q VLAN trunk driver within the operating system of the virtual machine. The process of frame tagging takes place at the OS level, and neither the virtual nor physical switch applies tags to these frames. Consequently, there is no need to configure a virtual VLAN on the physical switch in this scenario. With this approach, a single guest system can belong to multiple virtual VLANs simultaneously, regardless of how many virtual network interfaces the virtual machine possesses. Nevertheless, this method does impose additional overhead on the guest system since it requires resources to add and extract tags from frames.

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Using this mode requires trunking mode for the port on the physical switch, since different virtual machines may tag frames with different VLAN tags.

This method necessitates the installation of the 802.1Q VLAN trunk driver within the operating system of the virtual machine. The process of frame tagging takes place at the OS level, and neither the virtual nor physical switch applies tags to these frames. Consequently, there is no need to configure a virtual VLAN on the physical switch in this scenario. With this approach, a single guest system can belong to multiple virtual VLANs simultaneously, regardless of how many virtual network interfaces the virtual machine possesses.

Nevertheless, this method does impose additional overhead on the guest system since it requires resources to add and extract tags from frames.

3. Tagging mode at the physical switch level (External switch tagging, EST mode).

This setup is utilized for frame tagging by switches outside of ESX and does not necessitate frame tagging at the level of any ESX server elements. In this scenario, the physical switch automatically adds VLAN tags to frames originating from the ESX server. Therefore, trunking mode is unnecessary for the physical switch port.

In this mode of virtual machine tagging, the ESX Server host limits the maximum number of virtual VLANs that can be used to the number of physical network adapters.

Configuring VLANs in tagged mode at the physical switch level requires no additional configuration on ESX servers. Virtual switch port groups must either lack a VLAN ID or be set to 0 (also known as the Native VLAN).

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