IGMP (Internet Group Management Protocol) snooping can be enabled on member and translation VLAN (Virtual LAN)s so that multicast traffic can be monitored within the network.
IGMP snooping software examines all IGMP control traffic that enters the switch. IGMP control traffic received on a VLAN translation port is forwarded by the CPU to all other ports in the translation group. Software VLAN translation is performed on the packets which cross the translation boundary between member and translation VLANs. The snooping software detects ports joining and leaving multicast streams. When a VLAN translation port joins a multicast group, an FDB (forwarding database) entry is installed only on receiving a data miss for that group. The FDB entry is added for the requested multicast address and contains a multicast PTAG. When a VLAN translation port leaves a multicast group, the port is removed from the multicast list. The last VLAN translation port to leave a multicast group causes the multicast FDB entry to be removed.
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