Split-horizon

Split horizon is a concept that is used by distance vector dynamic routing protocols as a routing loop prevention mechanism. It simply states:

"Never send routing information back in the direction from which it was received."

So any routing information that was received via a particular interface will never be sent out that same interface.

EIGRP and RIP are among the routing protocols that use this mechanism.

Links:

https://networklessons.com/eigrp/troubleshooting-eigrp-route-advertisement

https://networklessons.com/rip/troubleshooting-rip