1. Introduction-15
1.2.9 IGMP Snooping and IP Multicast Filtering
The Internet Group Management Protocol (IGMP) runs between
hosts and their immediately neighboring multicast routers. The
protocol’s mechanisms allow a host to inform its local router that
it wants to receive transmissions addressed to a specific
multicast group.
Routers periodically query the LAN to determine if known group
members are still active. If there is more than one router on the
LAN performing IP multicasting, one of the routers is elected
“querier” and assumes the responsibility of querying the LAN for
group members.
Based on the group membership information learned from the
IGMP, a router can determine which (if any) multicast traffic
needs to be forwarded to each of its “leaf” subnetworks.
Multicast routers use this information, along with a multicast
routing protocol, to support IP multicasting across the Internet.
IGMP provides the final step in an IP multicast packet delivery
service since it is only concerned with the forwarding of multicast
traffic from the local route to group members on directly attached
subnetworks.
Routing switches support IP Multicast Filtering by:
l
Passively snooping on the IGMP Query and IGMP Report
packets transferred between IP Multicast Routers and IP
Multicast host groups to learn IP Multicast group members,
and
l
Actively sending IGMP Query messages to solicit IP Multicast
group members.
