Section 2.3: VLAN Trunking
When using VLANs in networks that have multiple interconnected switches, you need to use VLAN trunking between the switches. With VLAN trunking, the switches tag each frame sent between switches so that the receiving switch knows to what VLAN the frame belongs. End user devices connect to switch ports that provide simple connectivity to a single VLAN each. The attached devices are unaware of any VLAN structure.
A trunk link can transport more than one VLAN through a single switch port. A trunk link is not assigned to a specific VLAN. Instead, one or more active VLANs can be transported between switches using a single physical trunk link. Connecting two switches with separate physical links for each VLAN is also possible. In addition, trunking can support multiple VLANs that have members on more than one switch.
Cisco switches support two trunking protocols, namely, Inter-Switch Link (ISL) and IEEE 802.1Q.
2.3.1: Inter-Switch Link (ISL)
Cisco created ISL before the IEEE standardized a trunking protocol. Thus, ISL is a Cisco proprietary solution and can be used only between two Cisco switches. ISL fully encapsulates each original Ethernet frame in an ISL header and trailer. The original Ethernet frame inside the ISL header and trailer remains unchanged.
The ISL header includes a VLAN field that provides a place to encode the VLAN number. By tagging a frame with the correct VLAN number inside the header, the sending switch can ensure that the receiving switch knows to which VLAN the encapsulated frame belongs. Also, the source and destination addresses in the ISL header use MAC addresses of the sending and receiving switch, as opposed to the devices that actually sent the original frame.
2.3.2: IEEE 802.1Q
After Cisco created ISL, the IEEE completed work on the 802.1Q standard. IEEE 802.1Q uses a different style of header to tag frames with a VLAN number than the ISL. It does not encapsulate the original frame, but adds a 4-byte header to the original Ethernet header. This additional header includes a field with which to identify the VLAN number. Because the original header has been changed, IEEE 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer, because the FCS is based on the contents of the entire frame. IEEE 802.1Q also introduces the concept of a native VLAN on a trunk. Frames belonging to this VLAN are not encapsulated with tagging information. In the event that a host is connected to an IEEE 802.1Q trunk link, that host will be able to receive and understand only the native VLAN frames.