/mcpThe switch intercepts the IGMP Membership report that was sent by the host that wanted to join the group. 2. The switch creates a multicast entry for that group and links it to the port on which it has received the report and to all router ports. 3. The switch forwards the IGMP report to all router ports. This is so the router also receives the IGMP report, and updates its multicast routing table accordingly. 4. Scenario B: Host B now is the second host to join the same group. Host B sends an unsolicited IGMP Membership report. 1. The switch intercepts the IGMP Membership report sent by the host that wants to join the group. 2. The switch does not necessarily forward the IGMP report to all router ports. Actually, the switch forwards IGMP reports to router ports using proxy reporting, and only forwards one report per group within 10s. 3. Note: In order to maintain group membership, the multicast router sends a IGMP query every 60 seconds. This query is intercepted by the switch, and forwarded to all ports on the switch. All hosts that are members of the group answer that query. But, given the fact that the switch intercepts the reply report as well, the other host does not see each of the other reports, and thus, all hosts send a report (instead of one per group). The switch then uses Proxy Reporting as well, to forward only one report per group among all received responses. Assume Host A wants to leave the group, but Host B still wants to receive the group. The switch captures the IGMP Leave message from Host A. G The switch issues a group-specific IGMP Query for the group on that port (and only on that port). G If the switch does not receive a report, it discards this port from the entry. If it receives a response from that port, it does nothing and discards the leave. G Host B is still interested by that group on that switch. This would not be the last non-router port in the entry. Therefore, the switch does not forward the Leave message. G Now, assume Host B wants to leave the group and Host B is the last user interested by this group in this segment. The switch captures the IGMP Leave message from Host A. G The switch issues a group-specific IGMP Query for that group on that port. G If the switch does not receives a report, it discards this port from the entry. G This is the last non-router port for that GDA. The switch forwards the IGMP Leave message to all router ports and removes the entry from its table. G IGMP / CGMP Interaction In some networks, due to hardware limitations, you might not be able to run IGMP snooping on all switches. In this case, you might need to run CGMP on some switches in the same network. Note that this is a special case. The switch running IGMP snooping detects CGMP messages and detects that some switches in the network are running CGMP. Therefore, it moves to a special IGMP-CGMP mode and disables the proxy reporting. This is absolutely necessary for the proper operation of CGMP, because routers use the source MAC address of the IGMP report in order to