Black Box LBG1000A, Руководство пользователя

Страница: 21 / 142

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:
•
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.