The 802.1Q and ISL VLAN Trunking Protocols
Cisco has supported two different trunking protocols over the years: Inter-Switch Link (ISL) and IEEE 802.1Q. Cisco created the ISL long before 802.1Q, in part because the IEEE had not yet defined a VLAN trunking standard. Years later, the IEEE completed work on the 802.1Q standard, which defines a different way to do trunking. Today, 802.1Q has become the more popular trunking protocol; in fact, the Nexus series of switches supports only 802.1Q, and not ISL.
Both ISL and 802.1Q tag each frame with the VLAN ID, but the details differ. 802.1Q inserts an extra 4-byte 802.1Q VLAN header into the original frame’s Ethernet header, as shown at the top of Figure 9-6. As for the fields in the 802.1Q header, ...
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