To keep track of frames traversing a switch fabric, VLAN identification is used to identify which frames belong to which VLANs. There are multiple trunking methods:
Inter-Switch Link (ISL)
Proprietary to Cisco switches, it is used for FastEthernet and Gigabit Ethernet links only. Can be used on a switch port, router interfaces, and server interface cards to trunk a server. This server trunking is good if you are creating functional VLANs and don’t want to break the 80/20 rule. The server that is trunked is part of all VLANs (broadcast domains) simultaneously. The users do not have to cross a layer-3 device to access a company-shared server.
IEEE 802.1q
Created by the IEEE as a standard method of frame tagging. It actually inserts a field into the frame to identify the VLAN. If you are trunking between a Cisco switched link and a different brand of switch, you have to use 802.1q for the trunk to work.
LAN emulation (LANE)
Used to communicate multiple VLANs over ATM.
802.10 (FDDI)
Used to send VLAN information over FDDI. Uses a SAID field in the frame header to identify the VLAN. This is proprietary to Cisco devices.
Inter-Switch Link (ISL) Protocol
Inter-Switch Link (ISL) is a way of explicitly tagging VLAN information onto an Ethernet frame. This tagging information allows VLANs to be multiplexed over a trunk link through an external encapsulation method. By running ISL, you can interconnect multiple switches and still maintain VLAN information as traffic travels between switches on trunk links. ISL provides a low-latency, full wire-speed performance over FastEthernet using either half- or full-duplex mode.
Cisco created the ISL protocol, and therefore ISL is proprietary in nature to Cisco devices only. If you need a non-proprietary VLAN protocol, use the 802.1q, which is covered in the CCNP: Switching Study Guide.
ISL is an external tagging process, which means the original frame is not altered but instead encapsulated with a new 26-byte ISL header. It also adds a second 4-byte frame check sequence (FCS) field at the end of the frame. Because the frame is encapsulated with information, only ISL-aware devices can read it. Also, the frame can be up to 1522 bytes long. Devices that receive an ISL frame may record this as a giant frame because it is over the maximum of 1518 bytes allowed on an Ethernet segment.
On multi-VLAN (trunk) ports, each frame is tagged as it enters the switch. ISL network interface cards (NICs) allow servers to send and receive frames tagged with multiple VLANs so the frames can traverse multiple VLANs without going through a router, which reduces latency. This technology can also be used with probes and certain network analyzers. It makes it easy for users to attach to servers quickly and efficiently, without going through a router every time they need to communicate with a resource. Administrators can use the ISL technology to include file servers in multiple VLANs simultaneously, for example. It is important to understand that ISL VLAN information is added to a frame only if the frame is forwarded out a port configured as a trunk link. The ISL encapsulation is removed from the frame if the frame is forwarded out an access link.
Trunking
Trunk links are 100- or 1000Mbps point-to-point links between two switches, between a switch and router, or between a switch and server. Trunked links carry the traffic of multiple VLANs, from 1 to 1005 at a time. You cannot run trunked links on 10Mbps links. Trunking allows you to make a single port part of multiple VLANs at the same time. The benefit of trunking is that a server, for example, can be in two broadcast domains at the same time. This will stop users from having to cross a layer-3 device (router) to log in and use the server. Also, when connecting switches together, trunk links can carry some or all VLAN information across the link. If you do not trunk these links between switches, then the switches will only send VLAN 1 information by default across the link. All VLANs are configured on a trunked link unless cleared by an administrator by hand. Cisco switches use the Dynamic Trunking Protocol (DTP) to manage trunk negation in the Catalyst-switch engine software release 4.2 or later, using either ISL or 802.1q. DTP is a point-to-point protocol that was created to send trunk information across 802.1q trunks.