Trunking and VLAN Identification

Setting up VLANs on a single switch is relatively simple. First you define different VLANs, and then make ports members of those VLANs. However, when you interconnect or link switches across a network (referred to as trunking), you’ll need a way for switches to know on what VLAN a frame belongs. There are two main types of trunk links, as described below.

Access Link. When a link connects a single VLAN between switches, and no traffic for other VLANs is passed over that link, it is considered an access link. The only traffic that moves across an access link is traffic belonging the VLAN defined for the ports that are connected.

Trunk Link. If a link connects two switches, and the switches have 2 or more VLANs defined, it wouldn’t make much sense to set up a separate access link for each VLAN. Instead, it would be great if we could have traffic from multiple VLANs move across a single link. If a VLAN identification (frame tagging) technique is used, this is possible. The link is then known as a trunk link.

Consider the figure below, which outlines both access and trunk links.

Figure: Access and Trunk Links.

Remember that switches are always connected together using a crossover cable.

If you remember back to Chapter 2, none of the Ethernet frames we looked at had any field used to identify the VLAN membership of frame. In order for VLANs to work properly between switches, we’ll need some way to be able to let switches know what VLAN a frame is meant for.

Enter frame tagging. Frame tagging is a technique where additional VLAN identification information is added to a frame. Two main protocols exists for the purpose of Ethernet frame tagging – Inter Switch Linking (ISL) and IEEE 802.1q. Both modify a frame in different ways to add VLAN identifiers. Once implemented, VLAN tagging allows ports on the same VLAN (but on different switches) to communicate as though they were part of a single physical switch.

Adding more information to a frame creates a slight dilemma. Remember that an Ethernet frame has a maximum size of 1518 bytes. How can we add information to a large frame without making it appear oversized and thus invalid to network devices? Well, we need to configure the ports that link switches to use a VLAN identification protocol. When configured with VLAN tagging, a switch port will tag a frame with VLAN information when sending it out a trunk port. This tagging will be stripped away by the switch at the receiving end of the link. In this way, end devices need not be aware that any special framing or tagging took place. It also helps avoid end systems seeing these frames as being invalid. A VLAN tagged frame has a maximum size of 1522 bytes. The figure below illustrates the process by which a frame is tagged to include VLAN identification information. Note that the special tagging is added before it leaves the Switch 1 trunk port, and is removed once it enters the trunk port on Switch 2.

Figure: Frame tagging over a trunk link.

A number of different protocols exist for the purpose of adding VLAN identification to frames. These include:

InterSwitch Link (ISL). ISL is a Cisco proprietary VLAN identification protocol that can be used only on Fast Ethernet and Gigabit Ethernet trunk ports. Because the protocol is proprietary, it can only be used to trunk between Cisco devices. ISL actually re-encapsulates the entire original frame with a new header and a new CRC value.

IEEE 802.1q. The IEEE 802.1q is the industry standard method of VLAN identification. This protocol doesn’t entirely re-encapsulate a frame, but instead adds VLAN identification information into Ethernet frames. This in turn can make Ethernet frames as large as 1522 bytes. When you want to use VLAN identification on a network that includes equipment from different vendors, 802.1q should be used.

Dynamic Trunking Protocol (DTP). An enhancement of Cisco’s Dynamic ISL (DISL) protocol, DTP dynamically negotiates both ends of a trunk link to use a common VLAN identification protocol, such as ISL or 802.1q.

FDDI 802.10. While trunking protocols such as ISL are meant to create a trunk link between only two switches, 802.10 encapsulation allows VLAN tagging to be used on a shared FDDI backbone. It does this by adding a 4-byte Security Association Identifier (SAID) field to the FDDI frame header.

ATM Lane. When Ethernet or Token Ring networks connect over ATM, LAN Emulation (LANE) must be used to emulate their native environments (since ATM doesn’t support broadcasts, for example). In cases where VLANs are required over ATM connections, Emulated LANs (ELANs) need to be defined. Each ATM ELAN maps to a single VLAN.

Tip: VLAN tagging methods like ISL allow VLAN membership information to be transported with a frame across trunk links.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.