split horizon

[andrewjreid]

could someone explain to me how split horizon works with frame relay?

 

[mildmanrd]

By definition, split horizon will block routing updates from being sent out of the same interface it learned the update to begin with. In frame relay,you can have multiple networks connected to the same 'physical' interface. Without using subinterfaces(logical interfaces of the physical) in frame relay, the attached networks will not learn of each other because they would violate split horizon by learning an update from network A on serial0 and trying to update network B out of the same physical interface. With subinterfaces, cisco IOS treats these as seperates logical interfaces so split horizon will not be violated. If network A is on serial0.1 and network B is on serial0.2 then routing updates can occur because they are treated like seperate interfaces even though they are handled by the same physical interface. I don't know if this helped any, if not, maybe someone elso could explain it better.

DLB