| 1. | Every 60 seconds, one router (the querier) on each network segment sends a query to the all-hosts address (224.0.0.1) to see if there is at least one host still interested in receiving the multicast group. |
| 2. | In version 2, when a host decides to leave a group it has joined it sends a Leave Group message to the all-routers address (224.0.0.2). All routers on the local segment take note, and the querier router responds with a group-specific query message (sent to the group address), asking if others are still interested in receiving that group. Interested hosts must reply with a membership report, or the querier assumes there is no need to continue forwarding the group on that segment. |
| 3. | IGMPv3 adds support for multicast source filtering. |
| 4. | A router running IGMP can coexist with routers running older versions. To do this, the router must degrade itself to the highest common IGMP version in each multicast group. |
| 5. | CGMP is a protocol that runs between switches and routers. When routers receive IGMP messages, they pass the MAC of the requestor and the multicast group requested on to the switch. The switch then cross-references the requestor with its MAC table and enables the multicast group for a port. |
| 6. | IGMP snooping enables the switch to recognize IGMP and act on it as the IGMP packets cross the switch. This allows the switch to understand the requested multicast group and the port of the requestor. |
| 7. | All routers start as queriers and transition to non-queriers if they hear another querier with a higher IP address. |
| 8. | Router# show ip igmp group |
| 9. | IGMPv3 (RFC 3376) adds support for multicast source filtering and is backward compatible with v2 and v1. |
| 10. | show ip igmp interface |
