If IPv6 is disabled, MRTG and cfgmaker should behave in exactly the same way as previous versions. So the addition of IPv6 support should have no effect on existing MRTG setups unless IPv6 is enabled.
IPv6 support requires the Socket6 and INET6 libraries (see below). If MRTG can't find them, IPv6 is disabled.
For example, a target could be specified as:
Hostnames work as expected: first an IPv6 name lookup is tried, then an IPv4 lookup.
To query these targets, use the IPv4Only per-target option, which tells MRTG not to use SNMP over IPv6 for the target.
MRTG does not fall back to IPv4 for performance and correctness reasons. If there are many routers to query, a timeout for every router would make MRTG take too long to query them all. And if, for some reason, IPv6 connectivity to the target is lost, MRTG's error messages can help figure out what is wrong.
cfgmaker does fall back from IPv6 to IPv4. If IPv6 is enabled and cfgmaker is given a hostname that resolves to both an IPv6 and an IPv4 address, it first tries to query the target over IPv6. If it receives no answer, it tries again using IPv4. If the target answers, cfgmaker sets the IPv4Only option in the generated config file.
If you use Debian, you will need the packages libsocket6-perl and libio-socket-inet6-perl, which are (or should soon be) in unstable.
So far, IPv6 support has been tested on Linux only, and only with Socket6 version 0.12. Also note that IPv6 won't work at all if you don't have INET6.pm version 2.00 or newer.
perl Makefile.PL make
su make install
If you have installed the libraries successfully, cfgmaker and mrtg should automatically detect them and allow IPv6 support to be enabled.