The split horizon rule is a loop avoidance technique that prevents a route from being advertised out the interface through which it was learned.
By default, split horizon is enabled on all interfaces (except interfaces on which Frame Relay or Switched Multimegabit Data Service (SMDS) encapsulation is enabled).
On Routing Information Protocol (RIP) routers that have interfaces configured with secondary addresses, the split horizon behavior varies depending on whether the secondary IP is in the same subnet as the primary.
For information on split horizon behavior on RIP routers configured with secondary addresses refer to:
How Split Horizon Affects RIP/IGRP Routing Updates when Secondary Addresses Are Involved
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