Which command to only output the IPv6 routing table?
To output only IPv6 info, use the -f address_family option.

Example: netstat -nr -f inet6

From the netstat man page:


  −f address_family
  
  Limit statistics or address control block reports to those of the
  specified address family. The following address families are
  recognized: inet, for AF_INET, inet6, for AF_INET6 and unix, for
  AF_UNIX.