Core issue
The Internet Group Management Protocol (IGMP) querier feature enables IGMP snooping within a VLAN, where Protocol Independent Multicast (PIM) and IGMP are not configured because the multicast traffic does not need to be routed.
When IGMP querier is configured for a VLAN, the switch sends out IGMP general query messages every 125 seconds and listens for general query messages from other switches. If the switch receives a general query, a querier election starts. A querier election across switches is based either on an IP address or a MAC address.
For an inbound query, if the source IP address is nonzero, the election is based on the IP address and the switch with the lower source IP address becomes the querier. If the source IP address is zero for an inbound query, then the election is based on the source MAC address, and the switch with the lower MAC address wins the election and becomes the querier. The switch that becomes the non-querier maintains an "other querier interval" timer. When this timer expires, the switch elects itself as the querier.
Note: You must enable IGMP querier for IGMP snooping to work correctly in a VLAN in which no multicast routers are present.
Resolution
Upgrade to Catalyst OS (CatOS) version 8.x, which allows you to configure the source IP on a per-VLAN basis that the querier should use for multicast IGMP queries. Configure the source IP on a per-VLAN basis by issuing the set igmp querier address vlan ip_addr command.
Note: Prior to CatOS 8.x for Catalyst 6500 series switches, there was no way to configure the source IP address on a per-VLAN basis. Since the IGMP querier needs a source IP, it used the source IP address as 0.0.0.0.
