LB9030A-R2_LB9031A-R2_LB9032A-R2
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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” sub-networks.
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 sub-networks.
Routing switches support IP Multicast Filtering by:
•
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
•
Actively sending IGMP Query messages to solicit IP
Multicast group members.
The purpose of IP multicast filtering is to optimize a switched
network’s performance, so multicast packets will only be
forwarded to those ports containing multicast group hosts
