Split-horizon refers to the action taken by a router to avoid advertising a route back to the peer from which it was received. By default, SR OS applies split-horizon behavior only to routes received from IBGP non-client peers, and split-horizon only works for routes to non-imported routes within a RIB. This split-horizon functionality, which can never be disabled, prevents a route learned from a non-client IBGP peer to be advertised to the sending peer or any other non-client peer.
To apply split-horizon behavior to routes learned from RR clients, confed-EBGP peers or (non-confed) EBGP peers the split-horizon command must be configured in the appropriate contexts; it is supported at the global BGP, group and neighbor levels. When split-horizon is enabled on these types of sessions, it only prevents the advertisement of a route back to its originating peer; for example, SR OS does not prevent the advertisement of a route learned from one EBGP peer back to a different EBGP peer in the same neighbor AS.