ISL - Inter Switch Link

Cisco proprietary protocol for trunking links that carry multiple VLANs. Encapsulates the ethernet frame (contrary to 802.1Q) with a header and a checksum.

Adds another 22 Bytes to each frame, therfore slighty less efficient than 802.1Q.

Was introduced by Cisco before the final 802.1Q, is now slowly being replaced by this standard. Several Cisco switches support both protocols.

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