This sets, when acting as an IGMP client, which IGMP protocol version, v1 or v2, the modem uses when sending unsolicited membership reports, that is, joining a group without being asked. IGMPv1 and v2 have different report and leave message semantics, so this needs to match what the upstream querier, often at the hub, actually expects, since a mismatch can affect how quickly group membership changes, like leaving a group, are recognized. Setting this to v2(2) lets the modem send explicit IGMP Leave messages when a group is no longer needed, something IGMPv1 does not support, relying instead on the querier timing the group out.