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cisco ccna kit.Before going more deeply into the problems that exist in an NBMA environment, let's look at some of the topologies that can be used when connecting devices using VC; Table 25.5 lists the terms used to describe these topologies. The bottom half of Figure 25.10 shows a fully meshed network in which it is easy to simulate a broadcast environment: the router replicates the local broadcast and sends it through each VC on the interface. For example, in Figure 25.10, when Router A wants to send a local broadcast, it sends it through two VCs (to Router B and Router C, respectively). In a fully meshed environment, each device will receive the original broadcast frame. In this example, the same is true when Router B or Router C generates a broadcast.Chapter 25 Frame Relay 703Table 25.5 NBMA Topology Topology DescriptionFully-mesh Routers have VC connections to all other routersPartially-meshed Router has VC connections only to some (not all) other routersPoint-to-point Router has VC connections to only one other router (used to simulate leased line / private circuit connections)Star Sometimes referred to as central-branch topology, where routers are partially interconnected: each remote site router has connections to the central site router25.4.2 Horizontal Partitioning ProblemsThe main problem with NBMA becomes apparent when a partial mesh topology is used: it poses a problem for routing protocols that support horizontal segmentation. As mentioned in Chapter 14, distance vectoring protocols such as RIP use horizontal partitioning to prevent routing loops. Horizontal segmentation means that after routing information is learned on an interface, it cannot be propagated out through that interface.For partially meshed networks using VCs, this is a problem; for example, two routers may belong to the same subnet, but there is no VCo between them For partially meshed networks, horizontal partitioning rules may introduce routing problems, as shown in Figure 25.11. In the network shown in this figure, router A has VCs to three other routers,but for these three routers to communicate with each other, they must go through router A (central. Branch/star topology). Here it is assumed that all routers are located in the same subnet and that Router A uses the same serial interface to terminate its three