Yes, split horizon is enabled by default in EIGRP.
Because split horizon blocks route information from being advertised out an interface from which the route was learned you may well need to disable it in a frame-relay setup where the hub is using subinterfaces to connect to the spokes.
- if the hub router is using point to point subinterfaces for each spoke router, then there is no need to turn off split horizon for EIGRP. The hub will communicate with each spoke just fine and will easily advertise to a spoke the routes from the other spoke.
- if the hub router is not using point to point subinterfaces (is either using the physical interface or is using a multipoint subinterface) then there is need to disable split horizon for EIGRP on the hub router. This will allow it to advertise to a spoke the routes from the other spoke.
I want to clarify s'thing and thats split-horizon is only an issue if ur using an hub- and-spoke network with distance vector protocols which are Rip, Rip v2 and IGRP. Eigrp is an hybrid protocol so split-horizon shouldn't be an issue here.
And it's better to use subinterfaces instead of turning spilt-horizon off.
You are not correct about EIGRP and split horizon. It is true that EIGRP is sometimes described as a hybrid protocol, but that does not mean that split horizon is not an issue. I know from experience that split horizon is very much an issue in EIGRP.
I agree that it is best practice to use point to point subinterface for Frame Relay. But if you are in a situation where you are using a multipoint interface for hub and spoke and are using EIGRP then split horizon will be an issue to deal with.
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