What is difference between the 802.1q & ISL


  • ISL is an Cisco proprietary protocol.
  • Supports up to 1000 Vlans
  • Original frame is encapsulated and a new header is inserted during encapsulation process.
  • A 26 byte header and a 4 byte FCS (frame check sequence) are inserted. Hence a total of 30 Bytes of overhead.
  • ISL tags frames from native Vlans.
  • ISL is less preferred in networks because of its high overhead value which is added to each Ethernet frame.


  • It is an IEEE Standard.
  • 802.1q supports 4096 Vlans.
  • In 802.1q encapsulation process, a 4 byte tag is inserted into original frame and FCS (Frame Check Sequence) is re-calculated.
  • 802.1q does not tag frames from native Vlans.

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