KTI KS-2300, Руководство пользователя

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Notes: IGMP Protocol (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 router to group members on directly attached
subnetworks. The Intelligent Switch 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 members and routers instead of flooding to
all ports in the subnet (VLAN). The Intelligent Switch with IP multicast
filtering/switching capability not only passively monitor IGMP Query and
Report messages, DVMRP Probe messages, PIM, and MOSPF Hello
messages; they also actively send IGMP Query messages to learn locations
of multicast routers and member hosts in multicast groups within each
VLAN. Note, however, IGMP neither alters nor routes any IP multicast
packets. Since IGMP is not concerned with the delivery of IP multicast
packets across subnetworks, an external IP multicast router is needed if IP
multicast packets have to be routed across different subnetworks.