The set of ports that joining a multicast group. It has
the information of up to 2k ports with the port number
starting from the information indicated in
cisMcastPortRangeIndex object of the same row. For example,
if the value of cisMcastPortRangeIndex is 'twoKto4K', the
port number indicated in this object starting from 2049
and ending to 4096.
A port number is the value of dot1dBasePort for the port in
the BRIDGE-MIB (RFC 4188).