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  • #2224959

    port based vlan and 802.1g tag vlan


    by owen_cktai ·

    Hi , i am comfusing for the vlan 802.1q vlan and port based vlan. i dont know what the different between them. hope you can give me a hand.

    1.)Port-based VLAN assigns VLAN memberships to certain ports of a switch.

    2.)802.1q tag VLAN creates a TAG on each packet instructing ports of switch to accept and reject those packets.

    3.)example : 1st switches is have configure port based VLAN with VLAN 1 , VLAN 2 .
    2nd switches is have configure port based vlan with vlan 3 , vlan 4.

    4.)if i plug in a ethernet cable to 2nd switches, is possible i can be a member for vlan 1?

    This only can be done by 802.1q vlan.

    am i right, please advice.

    i need your hand to correct my fault.

All Answers

  • Author
    • #2634310


      by owen_cktai ·

      In reply to port based vlan and 802.1g tag vlan


    • #2611124


      by networker4 ·

      In reply to port based vlan and 802.1g tag vlan

      In regards to Vlans, a switch port can be one of two things. First, it can be defined as an access port that is statically assigned to a particular Vlan. In this mode, any traffic that enters or exits the port does not have a vlan tag. The Vlan to which that port is assigned in the switch configuration dictates which vlan this traffic belongs to. This is what you are referring to as a port based Vlan.

      The other possibility for a switch port is that it is configured as a trunk. A trunk port carries traffic for multiple Vlans over the same physical connection. A typical usage for trunk ports is to interconnect switches. In order for devices on either end of the link to determine what Vlan any particular frame belongs to, an 802.1q Vlan tag is added to each frame. This is what you are referring to as 802.1q Vlan.

      So for example, assume that you have two switches connected together, and that each switch has multiple Vlans assigned to it’s various ports. A client is attached to a port on switch A that is assigned to Vlan 30, and it needs to communicate to a server on switch B which is also connected to Vlan 30. The client doesn’t know anything about Vlans, so it sends a normal untagged ethernet frame to the server MAC address. Upon receiving this frame, the switch immediately tags the frame with an 802.1q header identifying the frame as belonging to Vlan 30. It then looks up the destination MAC address (the address of the server) and determines that that it needs to be sent out over a trunk port. Switch B receives this frame and again looks up the destination MAC address to determine the destination interface. In this case it determines that the destination interface is a local switch port. It strips the 802.1q header and sends the frame on to the server.

      So, to address your 4 points above:
      1) Correct
      2) Not really. The tags don’t tell the receiving switch whether or not to accept a frame, they just identify which Vlan the frame belongs to.
      3 and 4) No. If the second switch does not have any ports assigned to Vlan 1, it can not be a member of Vlan 1.


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