The address of the source sending multicast traffic to the group
identified by vRtrIgmpIfGroupAddress.
If the value of vRtrIgmpIfGroupMode corresponding to this entry is
'include', the receivers are interested in receiving traffic
from this source.
If the value of vRtrIgmpIfGroupMode corresponding to this entry is
'exclude' and if the value of vRtrIgmpIfGrpSrcExpiryTime is '0',
the receivers are not interested in receiving traffic from this
source.