Mostly needed to reach the outside with IPv6 using an IPv6-enabled router on your link.
Usage:
# /sbin/ip -6 route add <ipv6network>/<prefixlength> via <ipv6address> ¬ [dev <device>]
Example:
# /sbin/ip -6 route add default via 2001:0db8:0:f101::1
Usage:
# /sbin/route -A inet6 add <ipv6network>/<prefixlength> gw ¬ <ipv6address> [dev <device>]
A device can be needed, too, if the IPv6 address of the gateway is a link local one.
Following shown example adds a default route through gateway 2001:0db8:0:f101::1
# /sbin/route -A inet6 add default gw 2001:0db8:0:f101::1