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  • Content created by Rene Molenaar (CCIE #41726)

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Forum Replies

  1. Dear René,

    Thank you very much for this great job !

    I have a couple of questions about double tagging :

    To be effective and exploited, this attack means that the attacker frame is not already in a port with a vlan other than the native vlan ?

    In this sentence : “When the switch receives the frame, it will remove the first 802.1Q tag and forwards the frame with the second 802.1Q tag on its trunk interface(s).” Why the switch remove the first tag when this one " enter " the trunk ? Removing tags occurs when the frame exit the trunk for being delivered to the dest

    ... Continue reading in our forum

  2. Hi Prince,

    Normally when a switch receives a tagged frame, it will remove the tag and then forwards it on access interfaces or other trunks. If it is sent on other trunk interfaces, then it will be tagged again.

    With VLAN hopping, the VLAN of the attacker has to be the same as the native VLAN on the trunk. Here’s what happens:

    1. The attacker sends a double tagged frame, an inner (20) and outer (1) VLAN tag. The outer tag matches the native VLAN of the trunk.
    2. The switch receives the double tagged frame, looks at the outer VLAN (1) tag and removes it.
    3. The switc
    ... Continue reading in our forum

  3. For sure René, it helps a lot

    Thank you again for the great job! Well done!


  4. Hi, Rene.

    I confess that this lesson be confused, since I know frames that are received by the switch on access ports are dropped. The SW1 FastEthernet0/1 interface should be configured in trunk mode for this attack to work, correct?

  5. Hi Stefanio,

    I understand this can be confusing. When you read about VLAN hopping, they usually talk about interfaces in access mode that accept frames with two VLAN tags. The first tag is ignored, the second one allows you to jump from one VLAN to another.

    The 3560 switch that I used didn’t like this at all…it does not accept a frame that is tagged if the interface is configured in access mode. I didn’t test it but older switches (or IOS images), or other vendors switches, might accept a double-tagged frame on an access mode interface, making VLAN hopping poss

    ... Continue reading in our forum

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