Cisco switch ports that are configured as trunk ports can allow the transit of traffic that belongs to different Virtual Local Area Networks (VLANs). For example, a trunk link might allow all VLANs to transit (the default), or it might be made more restrictive by only allowing a select few VLANs (a process known as 'VLAN pruning'). In order to ensure the orderly transmission of frames from different VLANs, trunk links need to use some sort of encapsulation. The two (2) choices on Cisco switching platforms are Inter-Switch Link (ISL) and the IEEE 802.1Q. ISL, which is Cisco proprietary, has certainly seen a lack of adoption to the point that Cisco, in newer IOS releases like IOS-XE, no longer offers ISL as a choice and defaults to the only available trunking encapsulation, 802.1Q (or 'dot1Q' as it is commonly referred to). In this post you should compare and contrast Cisco's ISL with the IEEE 802.1Q focusing on the differences in their approach to encapsulation in addition to postulating on why ISL has become a legacy encapsulation method giving way to 802.1Q as the dominant encapsulation method for trunk links.

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