help: Enable poison reverse for split-horizon
create: vtysh -c "configure terminal" -c "interface $IFNAME" \
          -c "ipv6 ripng split-horizon poisoned-reverse"

delete: vtysh -c "configure terminal" -c "interface $IFNAME" \
          -c "no ipv6 ripng split-horizon" -c "ipv6 ripng split-horizon"

commit:expression: ($VAR(../disable/) == ""); \
   "You cannot have 'split-horizon poison-reverse' enabled with 'split-horizon' disabled for $IFNAME"