rface called a subinterface. Subinterfaces allow VCs to terminate into separate logical interfaces within a single physical interface. With point-to-point links (T1s in this case), each router must be able to support two T1 interfaces. Routers that support two T1s are generally more expensive than single-T1 routers. Additionally, point-to-point T1s cost more than frame-relay T1 services, especially over long distances. The example in Figure 22-6 is relatively simple, but what about larger networks? Figure 22-7 shows two networks, each with six nodes. On the left is a fully meshed net- work using frame relay, and on the right is a fully meshed network using point-to-point 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 full mesh Figure 22-7. Six-node fully meshed networks With six nodes in the network, there must be five links on each router. With frame relay, this can be accomplished with a single T1 interface at each router, provided the bandwidth for all the links will total that of a T1 or less. When using point-to-point links, however, routers that can support five T1s are required. In addition to the hard- ware costs, the telecom costs for a network like this would be very high, especially over longer distances. 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) /2 = 15 links are required. Frame-Relay Network Design Oversubscription When designing frame-relay networks, care must be taken 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 virtual circuits within the link exceeds the 