Using Explicit Tagging on the Network Backbone

It should be obvious by now that most of the network adapter cards that exist in the world today were not created with explicit tagging in mind. Switches that use explicit tagging are used for the most part in large WANs, such as the Internet. However, LAN switches have evolved to the point that for most small LANs, a VLAN-unaware switch will serve just fine in a small network of a few hundred computers.

However, when you connect a large number of these VLAN-unaware LANs to the network backbone in a larger network, as in a college campus or a large business, the core switches that connect these LANs must handle a much larger amount of network traffic. Thus, it's typical to use explicit tagging for large, high-capacity network switches than it is in smaller LAN environments (see Figure 9.3).

Figure 9.3. Explicit tagging can be employed in the network backbone to connect LANs that are not capable of using explicit tagging.

Here, the core switches that use explicit tagging are able to forward packets they receive within their switched network that understands the tags applied by the edge switches. The edge switches, however, add tags to frames they receive from the VLAN-unaware workstations, based on implicit tagging rules the administrator has set up. They remove the tags when they output frames to the individual workstations that are VLAN-unaware. Of course, these edge switches could just as easily have other switches attached to them, cascaded until you reach the end workstation or other network device. In this illustration, separate workstations are used to simplify the example. The edge switches could be connected to other switches that further subdivide the network and could use implicit tagging to forward the frames they receive from the edge switches.

A technique known as Multi-Protocol Label Switching (MPLS) uses a similar tagging technique and is commonly used in the core routers (or I guess you can now call them switches) that form the heart of the Internet. You can read more about MPLS in Chapter 37, "Routing Protocols."

Within the core of switches that do understand tagging, switching is done at a fast pace. Although some people still debate the use of MPLS, it is now used as a major protocol in the Internet core switches, due to the lack of any other standard protocol that might be used to replace it.