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, the 7210 SAS 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. Split-horizon functionality, which can never be disabled, prevents a route learned from a non-client iBGP peer from being advertised to the sending peer or any other non-client peer.
To apply split-horizon behavior to routes learned from RR clients or eBGP peers, configure the split-horizon command in either the global BGP, group or neighbor contexts. 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, the software does not prevent the advertisement of a route learned from one eBGP peer back to a different eBGP peer in the same neighbor AS.