IPv6 has a lot more support for auto configuration than IPv4. But
for this auto configuration to work on the hosts of a network, the
routers of the local network have to run a program which answers
the auto configuration requests of the hosts.

On Linux this program is called radvd, which stands for Router
ADVertisement Daemon. This daemon listens to router solicitations (RS)
and answers with router advertisement (RA). Furthermore unsolicited
RAs are also send from time to time.

These RAs contain information, which is used by hosts to configure
their interfaces. This information includes address prefixes, the MTU of
the link and information about default routers.