On Tue, 27 Jan 2004 14:13:41 -0700
David Stevens [off-list ref] wrote:

Below is a patch that allows forcing of the IGMP version. The primary
use would be with an IGMP-snooping switch that doesn't grok IGMPv3
packet formats and when there are no IGMPv2 or IGMPv1 multicast
routers on the network. By forcing the IGMP reports to version 2 (or
version 1, for really old switches), Linux would send reports that the
switch can understand for multicast forwarding to the port.

Perhaps some sanity to prevent values other than 1 or 2 from being
set?