nected to each other. The two PVCs terminate into a single interface on Router A. Physically, each router is connected only to one of the provider's telecom switches. These switches are the entry and exit points into and out of the cloud. Router A is connected with a DS3, while Routers B and C are connected to the cloud with T1s. Logically, frame relay creates the illusion that the routers on the far sides of the PVCs are directly connected. Router B Frame Relay PVC Router A Telecom switch Router C Figure 22-1. Simple frame-relay WAN In reality, there may be many devices in the cloud, any of which may be forwarding frames to the destination. Figure 22-2 shows what the inside of the cloud might look like. Router B Router A Router C Figure 22-2. Frame-relay physical network Chapter 22: Frame Relay Frame relay functions in a manner similar to the Internet. When you send a packet to a remote web site, you don't really care how it gets there. You have no idea how many devices your packets may pass through. All you know is what your default gateway is; you let your Internet service provider worry about the rest. Frame relay is similar in that you have no idea what the intermediary devices are, or how they route your data. When using frame relay, there are a finite number of destinations, all specified by you and provisioned by your telecom provider. For frame relay to create the illusion of a direct connection, each end of the virtual circuit is given a layer-2 address called a data link control identifier (DLCI, pro- nounced dell-see). These DLCIs (and your data) are visible only to the telecom switches that forward the frames and to your routers. Other customers connected to the frame-relay cloud cannot see your DLCIs or your data. Your telecom provider will determine the DLCI numbers. Virtual circuits can be combined in such a way that multiple end points terminate to a single DLCI. An example of this type of design is shown in Figure 22-3. Each virtual circuit can also have its own DLCI on both sides. How you desig