-to-point T1s. With six nodes in the network, there must be five links on each router. With Frame Relay, you can accomplish this with a single T1 interface at each router, provided the bandwidth for all the links will total that of a T1 or less. When you're using point-to- point links, however, routers that can support five T1s are required. In addition to the hardware costs, the telecom costs for a network like this would be very high, especially over longer distances. Chapter 23: Frame Relay Router A Router D Router A Router D Router B Router E Router B Router E Router C Router F Router C Router F Frame Relay Point-to-point full mesh T1 1 full mesh Figure 23-7. Six-node fully meshed networks To figure out how many links are required to build a fully meshed network, use the formula N (N-1) / 2, where N equals the number of nodes in the mesh. In our example, there are six nodes, SO 6 (6 1) / links are required. Oversubscription When designing Frame Relay networks, you must take care to ensure that the total amount of bandwidth being provisioned in all CIRs within a physical link does not exceed the port speed. A CIR represents guaranteed bandwidth, and it is technically impossible to guarantee bandwidth beyond the port speed. A Frame Relay link is con- sidered oversubscribed when the total bandwidth of all the VCs within the link exceeds the port speed of the link. For example, having four PVCs each with a 512 Kbps CIR is possible on a T1, even though the total of all the CIRs is 2,048 Kbps. Some providers will allow this, while others will not. The burst rate has no bearing on the oversubscription of a link. You should always plan carefully to ensure that the CIRs of your PVCs total no more than the port speed of your physical link. I use spreadsheets to keep me honest, but any form of documentation will do. Often, simple charts like the one in Figure 23-8 are the most effective. There are no technical limitations preventing oversubscription. During the Internet boom, small ISPs often debated the ethics of overs