ll you know is what your default gateway is; Router B Frame Relay PVC Router A Telecom switch Router C Figure 23-1. Simple Frame Relay WAN Router B Router A Router C Figure 23-2. Frame Relay physical network 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 you use 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 VC is given a Layer-2 address called a data link control identifier (DLCI, pronounced Chapter 23: Frame Relay 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. VCs can be combined in such a way that multiple endpoints terminate to a single DLCI. An example of this type of design is shown in Figure 23-3. Each VC can also have its own DLCI on both sides. How you design the Frame Relay PVCs will depend on your needs. Router B DLCI 102 Router A DLCI 101 DLCI 103 Router C Figure 23-3. DLCIs in a Frame Relay network DLCIs are mapped to IP addresses in Frame Relay networks in much the same way that Ethernet MAC addresses are mapped to IP addresses in Ethernet networks. Unlike with Ethernet, however, which learns MAC addresses dynamically, you'll usually have to map DLCIs to IP addresses yourself. Ethernet networks use the Address Resolution Protocol (ARP) to determine how MAC addresses map to known IP addresses. Frame Relay uses a protocol called Inverse ARP to try to map IP addresses to known DLCIs. Frame Relay switches report on the statuses of all configured DLCIs on a regular basis. Be careful when configuring Frame Relay. There may be PVCs config- ured that you do not wish to enable, and Inverse ARP may enable those links 