I am using the Trainsignal DVD to study for the CCNA certification test. I just worked through the frame relay lab. I have three routers connected to a fourth router that is configured as a frame relay switch. The lab was quite interesting. One thing that I need some clarification on is the use of subinterfaces in frame relay.
The instuctor had us use a physical interface on the hub router, a multipoint subinterface on one of the spokes, and a point-to-point subinterface on the second spoke. Each serial interface had an ip address on the same subnet. He explained that all the different interfaces were not require for the lab, but that he wanted us to experience configuring each type. I presumed that the use of the various types of interface are proper to more advance study. None the less it would be nice to understand their context.
Could one configure three routers in a hub and spoke topology using all physical interfaces with frame relay maps? If so, when would subinterfaces be required? I understand that a multiport subinterfaces are required to connect to multiple networks. I did some reading on the web and found that subinterfaces are used to deal with a split-horizon problem. I did not understand what that meant. could someone explain that?