|  | 9.8 | The Benefits of Virtual LANs (VLANs)
| 9.8.4 | VLAN tagging
A normal, single link can be used to pass traffic between multiple subnetworks. However, when a single link is used to transmit multiple VLAN traffic, then the switch ports and/or router ports which connect the two devices must be able to do “tagging.” In Figure , the link between the two switches, those switch ports must be capable to do tagging. Again, tagging is needed between switches, or a switch and a router, to pass VLAN traffic, if a single link is used. Your switches must have ports that can do this trunking or tagging. Some of the different tagging protocols include: IEEE 802.1q ISL (Inter-Switch Link) - Cisco proprietary 802.10 - FDDI ATM LANE Some VLAN terminology: Tagging - used to Identify which VLAN a frame belongs to Trunk - a single link that carries multiple VLANs In Figure , you will notice that there are two links where tagging is taking place. Between Switch 1 (Port A) and Switch 2 (Port A) Between Switch 2 (Port B) and Router (Ethernet 0) The links between the switches, and the switch and the router where tagging is occurring is known as a trunk. A trunk is a link that carries multiple VLANs. (The term “trunk” also has other meanings in networking and telecommunications.) This is because these links are required to carry frames belonging to multiple VLANs, VLAN 1 and VLAN 2. Not all switches and routers (or Cisco IOS’s) are capable of doing the tagging, so it is important to make sure your have networking equipment that can perform the tagging. On some switches, there may only be a few ports that can be configured for tagging. These are usually the higher bandwidth ports (100 Mbps) on the switch. In Figure , these switch ports must be capable of tagging: Switch 1 : Port A Switch 2 : Port A and Port B Router : Ethernet 0 (Usually dependent upon the Cisco IOS) Question: What would you do if your equipment did not do tagging, but you still wanted to do VLANs? Answer: You would have to have a separate link for every VLAN between switches, and between a switch and a router. Be sure to configure the switch ports for the proper VLANs.