IP-COM F1226P, Руководство пользователя

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Principle of IGMP snooping
By listening to the conversations between hosts and routers, the switch maintains a map of which links need which
IP multicast streams. Multicast streams may be filtered from the links which do not solicit them. An
IGMP-Snooping-disabled layer-2 device will flood multicast traffic to all the ports in a broadcast domain (or the
VLAN equivalent). With IGMP snooping enabled, known multicast traffic will be forwarded to hosts that have
explicitly joined the group. It provides switches with a mechanism to prune multicast traffic from links that do not
contain a multicast listener (an IGMP client).
How IGMP Snooping Works
A switch that runs IGMP snooping performs different actions when receiving different IGMP messages.
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local subnet to determine
which active multicast group members exist on the subnet. After receiving an IGMP general query, the switch
forwards it through all ports in the VLAN (except the port that received the query) and performs corresponding
actions on the receiving port (resets/enables the age timer).
When receiving a membership report
A host sends an IGMP membership report to the multicast router in the following circumstances:
After receiving an IGMP query, a multicast group member host responds with an IGMP membership report.
When intended to join a multicast group, a host sends an IGMP membership report to the multicast router to
announce that it wants to join the multicast group. After receiving an IGMP membership report, the switch
forwards it through all the router ports in the VLAN, resolves the address of the reported multicast group and
performs corresponding actions on the receiving port (resets/enables the age timer). A switch does not forward an
IGMP membership report through a non-router port.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, so the switch
cannot know immediately that the host has left the multicast group. However, as the aging timer on the member
port that corresponds to the host expires, the the switch immediately deletes its forwarding entry from the
forwarding table.
When an IGMPv2 or IGMPv3 host leaves a multicast group, it sends an IGMP leave message to the multicast
router to inform of such leave.
When receiving an IGMP leave message from the last member port, the switch forwards it through all router ports
in the VLAN and resets the aging timer on the receiving port (the port that received the IGMP leave message)
instead of immediately deleting its corresponding forwarding entry from the forwarding table as it cannot know
whether there are still other members of that multicast group attached to such port.
After receiving the IGMP leave message from a host, the IGMP querier resolves the multicast group address in the
message and sends an IGMP group-specific query to that multicast group through the port that received the leave
message. After receiving the IGMP group-specific query, the switch forwards it through all its router ports in the
VLAN and all member ports for that multicast group.