Upgrading and Repairing Networks (5th Edition)

There currently are two standards on which many VLANs are based. The first is called IEEE 802.1D, and the second is called IEEE 802.1Q. For the most part, the newer standard builds on and extends the IEEE 802.1D standard. The basic difference is that the IEEE802.1D standard defines switches that are VLAN-unaware, whereas the newer IEEE 802.1Q standard provides for VLAN-aware switches.

The actual details of these standards are beyond the scope of this book, much less this chapter. However, there are a few details that should be covered so that you'll be more VLAN-aware when you make purchasing decisions:

• IEEE 802.1Q includes backward compatibility with switches that were based on the IEEE802.1D standard.

• The default behavior of a switch that is IEEE 802.1Q compliantin the absence of any association rulesis to function as though VLANs were based on ports. This does not mean that IEEE 802.1Q provides for simply port-based VLANs. It just means that it is the default if the vendor doesn't implement any other techniques for using a set of rules to create VLANs.

• The IEEE 802.1Q standard talks about how association rules should be processed, but it does not require that any particular kind of rule be implemented on a switch. It's more a guideline to be used if a vendor decides to use one or more rule-based mechanisms for creating VLANs.

• The IEEE 802.1Q standard applies only to switches, not to the creation of network adapter cards that are VLAN-aware. That means if you do purchase adapter cards that support explicit tagging, you should be sure that it's the same method used by your switch.

The IEEE 802.1Q standard adds a filtering database that can be used to map devices to certain ports, and it describes algorithms that can be used with this filtering database to determine which VLAN a frame is a member of. The earlier standard provided for a filtering database, but the newer standard expands on its functionality. Perhaps more important to an administrator in a large network, the standard defines a MIB (management information database) that can be used to manage switches.

For explicit tagging, the IEEE 802.1Q standard defines standard tag formats so that you don't have to worry about different tagging schemes used by proprietary solutions implemented in earlier switches by different vendors.

This standard also defines a priority mechanism, which is not really necessary for VLAN operation but was added because some protocols, such as Ethernet, don't have a mechanism for this. Although this is not specifically related to the concept of a VLAN, it was decided during the standards process to include a priority mechanism simply to avoid having to go back later and create another standard for prioritizing Ethernet frames. Strange, but true.

Another important concept is that the newer standard sets forth the methods used when a switch is used to connect dissimilar network typessuch as Ethernet and Token-Ring (and FDDI, though that technology is slowly becoming a dinosaur in today's market). This detail might not seem important, but there are many differences in the frame types used by these different network protocols (such as the big- or little-endian method of encoding bitsin other words, which bit that is transmitted for a byte is the most significant, and which is the least significant?). The newer standard defines the mechanisms for encapsulating dissimilar frames types within the frame type used on the network. For example, it tells which method to use for sending Token-Ring frames through an Ethernet network, and vice versa. Although it seems that Token-Ring has seen its better days, there are legacy systems out there that still use it. Using a switch that is IEEE 802.1Q compliant enables you to connect Token-Ring networks across a link that supports a faster technology, such as Gigabit Ethernet.

The final topic I'll touch on is the fact that IEEE 802.1Q also sets forth a protocol (called GVRP) that can be used by switches to exchange information about VLAN membership. This is perhaps one of its more important concepts. This means that you won't have to configure each switch in the network when VLAN membership is changed.

The IEEE standards cover a lot more territory than I can go into in this chapter. There are entire books on this subject, and if you are seriously considering implementing VLANs in a large network, you should pursue further reading, specifically purchasing the standards from the IEEE and using them to evaluate products to determine whether they meet the standards. However, for most networks today, as long as you use switches from the same manufacturer or from those who state that their products will interoperate with other vendors, you should not encounter many difficulties in setting up VLANs on your network.