n the frame- relay PVCs will depend on your needs. Router B DLCI 102 Router A DLCI 101 DLCI 103 Router C Figure 22-3. DLCIs in a frame-relay network Frame Relay 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 without your knowledge. The primary benefits of frame relay are cost and flexibility. A point-to-point T1 will cost more than a frame-relay T1 link between two sites, especially if the sites are not in the same geographic location, or LATA. Also, with a point-to-point T1, 1.5 Mbps of dedicated bandwidth must be allocated between each point, regardless of the utilization of that bandwidth. In contrast, a frame-relay link shares resources. On a frame-relay link, if bandwidth is not being used in the cloud, other customers can use it. The notion of shared bandwidth raises some questions. What if someone else is using the bandwidth, and you suddenly need it? What if you're using the band- width, and someone else suddenly needs it? Frame relay introduces the idea of the committed information rate (CIR), which helps address these concerns. Ordering Frame-Relay Service When you order a frame-relay link, telco needs to know four pieces of information to provision the circuit. One of these is the CIR. Here are the requirements: The addresses and phone numbers of the end points The street addresses-and, more importantly, phone numbers-in use at each end p