The value of vRtrMldIfGroupCompatMode is used in order for routers
to be compatible with older version routers. MLDv2 hosts MUST operate
in version 1 and version 2 compatibility modes. MLDv2 hosts MUST
keep state per local interface regarding the compatibility mode of
each attached network. A host's compatibility mode is determined
from the Host Compatibility Mode variable which can be in
one of two states: MLDv1 or MLDv2. This variable is
kept per interface and is dependent on the version of General Queries
heard on that interface as well as the Older Version Querier Present
timers for the interface.