vRtrIgmpIfGroupCompatMode is used in order for routers to be
compatible with older version routers. IGMPv3 hosts MUST operate
in version 1 and version 2 compatibility modes. IGMPv3 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 three states: IGMPv1, IGMPv2 or IGMPv3. 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.