Cisco® Field Manual: Catalyst® Switch Configuration

9-1. IGMP Snooping

• Some Catalyst switches can be configured to intercept IGMP join requests as hosts ask to join IP multicast groups.

• IGMP join requests can occur as the following happen:

  - Hosts send unsolicited membership reports to join specific multicast groups.

  - Multicast routers acting as IGMP queriers send IGMP membership query messages to the all-hosts multicast group 224.0.0.1 every 60 seconds. Interested hosts respond with membership reports to join specific multicast groups.

• The switch keeps a record of the IP multicast group, its Layer 2 MAC address, and the switch ports that connect to the requesting host and the multicast router.

• Multicast routers cannot keep a detailed list of all hosts belonging to a multicast group. Rather, a router ...