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cisco ccna kit ebay.VCoRouter DFigure 25.11 NBMA and horizontal segmentation issuesSwitchSwitch Virtual CircuitHere's a look at the problems posed by horizontal segmentation from a routing perspective, assuming that all routers are running RIPv2o Router B, Router C, and Router D have no problems-they all have only one VC through which routing updates can be sent and received; however, Router A will experience problems when propagating routing information from Router B, Router C, or Router A will experience problems when propagating routing information from Router B, Router C, or Router D to other routers in the subnet.For example, suppose that Router B generates a routing update. Since Router B does not have VCs to Router C and Router D, it forwards the update to Router A, expecting Router A to forward it to other routers on the subnet. However, after receiving a routing update from Router B, Router A cannot forward it to Routers C and D according to the horizontal partitioning rules. Although these two routers are connected through different VCs, these VCs are located on the same physical interface; therefore, by default, Router A cannot propagate routing information from the remote router to other remote routers.Figure 25.11 shows a simple example of an NBMA environment. Although it is possible to travel to each router in the subnet via an extra hop, broadcast and multicast do not work correctly.Keep in mind the problems associated with horizontal segmentation when running dynamic routing protocols, as shown in Figure 25.11. For example, in a central-branch topology, if the central router (Router A) can ping all other routers, but not between two branch routers (such as Router B and Router C), then it is likely that horizontal partitioning is causing this reachability problem.25.4.3 Solutions to Horizontal Partitioning Problems