Routers R1， R2， and R3 are configured in a hub and spoke frame relay environment， with router R1 as the hub. You have configured Router R1， Router R2， and Router R3 to run EIGRP over the frame relay connections. No sub-interfaces are used. You have configured a single IP subnet on all the Frame Relay interfaces. Router R1 can reach both router R2 and R3， but R2 and R3 can not reach each other. What is the probable cause of this problem？ 
正确答案:
The rule of split horizons cause this problem.Split horizon is enable by default on the interface that encapsulated with frame relay.
The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach that destination. Without sub-interfaces， split-horizon goes into effect， and all routes learned from the Serial interface will not be advertised out of that interface.
The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach that destination. Without sub-interfaces， split-horizon goes into effect， and all routes learned from the Serial interface will not be advertised out of that interface.
答案解析:有
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