ATL Telecom R1-SW Ethernet Switch - Manual (Page 236)

R1-SW24L2B User’s Manual
11-2
IGMP (Internet Group Management Protocol)
IGMP is used to dynamically register individual hosts in a multicast group on a particular LAN.
Hosts identify group memberships by sending IGMP messages to their local multicast router.
Under IGMP, routers listen to IGMP messages and periodically send out queries to discover
which groups are active or inactive on a particular subnet.
IGMP snooping manages multicast traffic at Layer 2 on the Corecess R1-SW24L2B by allowing
directed switching of IP multicast traffic. Switches can use IGMP snooping to configure Layer 2
interfaces dynamically so that IP multicast traffic is forwarded only to those interfaces
associated with IP multicast devices.
When IGMP snooping is enabled on the Corecess R1-SW24L2B, the route processor sends out
periodic general queries to all VLANs. The switch processor responds to the route processor’s
queries with only one join request per MAC multicast group. The switch processor creates one
entry per VLAN in the Layer 2 forwarding table for each MAC group from which it receives an
IGMP join request. All hosts interested in this multicast traffic send join requests and are added
to the port mask of this forwarding table entry.