In the world of switching, when you are trunking ports, you have two options: Inter-Switch Link (ISL) or IEEE 802.1q. Using either of the following trunking methods identifies the VLAN as they go across switches in your network.
Types of trunking
Inter-Switch Link (ISL)
This type of trunking is Cisco's proprietary trunking type; it is limited to only 1,000 VLANs, but still gets the job done. It will explicitly identify VLANs going across the trunk links, and has more overhead than the IEEE 802.1q trunking method since it tags the native VLAN and all other VLANs as well.
Today, some Cisco switches still support ISL trunking, but Cisco is moving away from ISL to the newer type of trunking; that is, 802.1q.
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