Back in Chapter 7 you looked at the concept of subinterface. Subinterfaces allow you to configure a single physical interface in such a way that it can act as multiple interfaces, each of which can have different properties. When configuring Frame Relay on a router, each virtual circuit is usually assigned to its own serial subinterface. This not only allows multiple virtual circuits to function across a single physical interface, but also allows the properties of each subinterface to be configured individually.
As an example, consider the figure below. In it, a company has one head office and three branch locations. Each of the branch offices connects to the head office location using a dedicated PVC. Router interface S0 at the head office is divided into 3 subinterfaces, each of which would be configured with an appropriate IP address and its associated DLCI number. In order to simplify the management of subinterfaces (and make things easier to remember), subinterfaces are usually given a number that maps directly to a PVC’s DLCI number. This is not a requirement, but rather an easy way to keep track of which PVC is associated with a given subinterface. For example, DLCI 23 at the head office would be configured on subinterface S0.23.
In order for a Frame Relay network to function correctly, interfaces will still need to be configured according to a logical addressing scheme. I’m going to assume that our network runs only IP in this example, but it could be running other Network layer protocols like IPX or AppleTalk as well. The issue at hand is how logical addressing should take place over the Frame Relay network. For example, each PVC could be treated as a point-to-point network and, as such, would require its own IP subnet. However, if multiple PVCs are connected into a single router in a hub-and-spoke type configuration, it’s also possible to configure the connections as a single subnet instead – this configuration is known as multipoint. Point-to-point connections are generally more common when connecting locations by Frame Relay, but either configuration is valid. Both are illustrated in the figure below.
Another important reason for the use of subinterfaces on a Frame Relay network relates to issues associated with distance vector routing protocols. If you’ll recall, split horizon prevents a router from sending routing table updates that were received on a given interface back out the same interface. As such, if you had the multipoint configuration shown in the figure below, where the head office location does not use subinterfaces, routing table updates from Router B could never reach Router C or vice versa. Quite simply, Router A would not be allowed to forward updates received from B to C over the same interface.
This is not an issue when subinterfaces are used, since the routing protocol will consider subinterfaces as distinct and separate. If subinterfaces were used on Router A in Figure 21, routing table updates could pass between all three routers via Router A without issue.