IPv6用RIPng（スプリットホライズン）

   Split Horizon is a algorithm for avoiding problems caused by
   including routes in updates sent to the gateway from which they were
   learned.  The basic split horizon algorithm omits routes learned from
   one neighbor in updates sent to that neighbor.  In the case of a
   broadcast network, all routes learned from any neighbor on that
   network are omitted from updates sent on that network.

   Split Horizon with Poisoned Reverse (more simply, Poison Reverse)
   does include such routes in updates, but sets their metrics to
   infinity.  In effect, advertising the fact that there routes are not
   reachable.  This is the preferred method of operation; however,
   implementations should provide a per-interface control allowing no
   horizoning, split horizoning, and poisoned reverse to be selected.

   For a theoretical discussion of Split Horizon and Poison Reverse, and
   why they are needed, see section 2.1.1 of [1].