VLAN Trunking: ISL and 802.1Q

VLAN trunking allows network devices to send  traffic for multiple VLANs over a single link.  To know what VLAN a frame belongs to, the sending switch or network device adds a header to the original Ethernet frame.  This header has a field included which defines the VLAN ID of which VLAN the frame is associated with.

Older networking devices are capable of differentiating VLANs in two ways: ISL and 802.1Q

Here are the differences between the two types of trunking protocols:

ISL

  • Supports Normal and Extended range VLANs
  • Protocol defined by Cisco
  • Encapsulates the original Ethernet frame
  • Does not understand the concept of Native VLAN

ISL

802.1Q

  • Supports Normal and Extended range VLANs
  • Protocol defined by IEEE
  • Inserts tag into existing Ethernet header
  • Understands the concept of a Native VLAN

802dot1q

The concept of the Native VLAN is that there are network devices that do not understand VLANs or VLAN trunking, so as they transmit data across a switched network they do so without encapsulating/tagging their traffic with a VLAN ID.  The default VLAN ID that handles untagged traffic traditionally is VLAN 1 however this can be configured to be something else if needed for any reason.

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