VLAN Trunking Protocols

Cisco proprietary protocol.
ISL tag each frame with the VLAN ID using 30-byte VLAN header.
ISL tags frames from native VLANs.
ISL encapsulation adds 30 byte high overload header to each Ethernet frame. Hence it is less preferred compared to 802.1Q encapsulation. Even Cisco stopped supporting ISL in some of the newer models of LAN switches.
In ISL, VLAN ID size is 15 bits.

IEEE standard protocol. Can be used by all devices from different manufacturers. 802.1Q tags each frame with the VLAN ID using a 4-byte header.
802.1Q does not add an 802.1Q header to frames in the native VLAN.
802.1Q encapsulation is the most commonly used protocol.
In 802.1Q, 12 bits is used to tag the VLAN ID.
802.1Q supports 4094 VLANs (Normal range – 1 to 1005, Extended range – 1006 to 4094).

Note: Native VLAN is the VLAN associated with all untagged traffic on a trunk.

To support connections to devices that do not understand trunking.

Example:

A Cisco switch could be connected to a switch that does not understand 802.1q trunking, then the Cisco switch send frames in the native VLAN. If the frame is not tagged with 802.1q header, then the other switch (without trunking port) would understand the frame.

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