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VLAN

Reference: [Book] Network Introduction for IT Engineers



1. What is VLAN?

  • A technology that logically divides and configures a LAN regardless of physical placement
    • A single Switch can be divided into multiple VLANs
      • They operate as separate switches
  • As departments in companies need network separation and numerous terminals like smartphones and PCs connect to networks, network segmentation is important
    • Why network segmentation is needed
      1. Performance degradation of terminals due to excessive broadcast
      2. For security enhancement and blocking purposes
      3. Applying policies based on service characteristics
  • When VLANs are separated, since a single device is logically divided into different networks, not only Unicast but also Broadcast cannot communicate between VLANs
    • If communication between VLANs is needed, it requires the help of Layer 3 devices since it is communication between different networks
  • Using VLANs, networks can be separated regardless of physical configuration, and terminals on physically different floors can be grouped into the same network using a single VLAN
    • Communication between separated terminals occurs through Layer 3 devices


2. Types of VLANs

There are port-based VLANs and MAC address-based VLANs for VLAN assignment methods


Port Based VLAN

  • When VLANs were first introduced, switches were expensive and played the role of bundling multiple hubs, so using the switch for multiple networks by dividing it was the purpose of applying VLAN functionality

    • Using the switch's ports in a logically divided manner is called Port Based VLAN
      • Most VLANs we generally refer to are Port Based VLANs
    • Regardless of which terminal connects, assigning a VLAN to a specific port on the switch causes it to belong to that assigned VLAN

  • In a switch configured with Port Based VLAN, the VLAN assignment criterion is the Switch's port

    • ex)
      • If PC AA connects to port 1, it belongs to VLAN 10; if connected to port 4, it belongs to VLAN 20

MAC Based VLAN

  • As user mobility increased, MAC Based VLAN based on MAC Addresses was developed

    • Using MAC Based VLAN, wired users remain in the same VLAN even when they move

  • Rather than assigning VLANs to fixed ports on the Switch, it assigns VLANs based on the terminal's MAC Address connecting to the switch

  • When a terminal connects, the switch recognizes the terminal's MAC Address and changes the port to the designated VLAN

    • Since VLAN information can change based on the terminal, it is also called Dynamic VLAN

  • The VLAN assignment criterion for MAC Based VLAN is the PC's MAC address

    • ex)
      • PC AA is assigned the same VLAN regardless of which switch or which port it connects to


3. How VLAN Works (Trunk/Access)

  • In Port Based VLAN, each Switch port is configured with a VLAN to use, and even if connected to a single switch, ports with different VLANs cannot communicate
    • Different VLANs mean being connected to separate, isolated switches, so inter-VLAN communication is impossible
      • Communication between different VLANs requires Layer 3 devices like routers
        • In networks segmented by VLANs, the broadcast ARP Request cannot be delivered to other VLANs, so communication must occur through Layer 3 devices
  • Separating networks by configuring VLANs on switch ports allows more efficient use of equipment compared to physically separating switches
    • VLAN separation creates the effect of multiple logical switches
  • When switches need to be interconnected in a situation with multiple VLANs, communicating between each VLAN requires as many ports as VLANs
    • A switch with divided VLANs is treated as physically separate switches
    • ex)
      • If one switch has 3 VLANs configured, 3 ports are needed for each VLAN to communicate between switches
        • In medium/large networks using more VLANs, connecting by VLAN wastes many ports just for device interconnections
    • The VLAN Tag feature was created to solve this problem!

Tagged Port (Trunk Port)

  • The Tag feature allows multiple VLANs to be transmitted together through a single port

    • This port is called a Tagged port or Trunk port

  • The Tagged port that must transmit multiple VLANs simultaneously inserts a VLAN Field in the middle of the Ethernet Frame during communication

    • When sending packets through the Tagged Port, a VLAN ID is attached, and the receiving side removes the VLAN ID and sends the packet to the corresponding VLAN

  • Using Tagged Ports allows bundling the multiple ports previously needed for each VLAN's communication into one, enabling flexible network design without port waste

  • With the Tagged Port feature added to switches, the MAC Address Table used for packet transmission also changed

    • A field specifying VLAN was added to the MAC Address Table to prevent communication between different VLANs!
      • When a network is separated using VLANs on a single switch, it operates as if MAC Address Tables exist per VLAN

  • Tagged Ports are used to deliver multiple VLANs, meaning multiple networks, through a single physical port

  • Tagged Ports are generally used for connections between switches that have multiple networks configured simultaneously

  • When packets enter through a Tagged Port, they are transmitted to the tagged VLAN side while stripping off the Tag


Untagged Port (Access Port)

  • Regular ports are called Untagged Port or Access Port

  • Untagged Ports are used only when belonging to a single VLAN

    • So typically, servers belonging to a single network are configured as Untagged

  • When packets enter through an Untagged Port, they are transmitted only to the same VLAN



Virtualized Servers

  • Even server-connected ports, not just switch-to-switch connections, may need to communicate with multiple VLANs when connected to virtualized servers
    • In this case, even server-connected ports are configured as Tagged rather than Untagged
      • Since it is in Tagged state, the interface on the virtualized server side must also be configured in tagged state
    • Since a virtual switch exists inside the virtualized server, it is easier to understand when viewed as a switch-to-switch connection


Inter-VLAN Communication

  • VLANs prevent unicast, multicast, and broadcast from crossing VLAN boundaries due to the switch communication segmentation function
  • Generally, different VLANs mean being divided into separate networks, so different networks with different IP address assignments is typical
    • If communication between different networks is needed, the help of Layer 3 devices like routers is required