ISL was defeated by 802.1q because of the lack of vlan?

Catalyst switches have supported only up to 1024 VLANs, with this number being derived from ISL, the 10-bit VLAN ID field.

On the flip side, 802.1Q includes a 12-bit VLAN ID field, which means supports up to 4096 VLANs.

Is this the main reason?

 

Comments

  • Castor,

    802.1Q  has less overhead than ISL; ISL adds an additional 30 bytes to a frame (26 byte header and 4 byte trailer) as opposed to an 4 byte tag added by 802.1Q.

     

     

    Catalyst switches have supported only up to 1024 VLANs, with this number being derived from ISL, the 10-bit VLAN ID field.

    On the flip side, 802.1Q includes a 12-bit VLAN ID field, which means supports up to 4096 VLANs.

    Is this the main reason?

     

     

  • Edwinrg00

    Thanks.

    Ah, less overhead is the reason.

  • One could also make the case that ISL is more resource intensive from the switch's point of view. ISL has to completely re-encapsulate the frame at layer 2. 802.1Q just modifies the frame by tagging without doing another full encapsulation.

     

    Of course all of this is done at the ASIC level so, it's really not a big deal. It's just more stuff to do.

  • mjk2374

    Thanks.

    "ISL has to completely re-encapsulate the frame"

    That figures.

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