IP-COM G3210P, Руководство пользователя

Страница: 83 / 138

78

Advanced Settings

How IGMP Snooping Works
A switch that runs IGMP snooping performs different actions when receiving different
IGMP messages.
1
．
Group query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the
local subnet to query which 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 receives the query) and performs corresponding actions on the receiving port
(mainly resets/enables the age timer on this port).
2
．
Report membership
After receiving an IGMP query, a multicast group member host responds with an IGMP
membership report. If the host wants to join a multicast group, it will send 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 (mainly resets/enables the age
timer). The switch does not forward an IGMP membership report through a non-router
port.

Leave the multicast group
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leaving
message. However, as the aging timer on the corresponding member port expires, 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 leaving
message to the multicast router to inform of such leave.
When receiving an IGMP leaving 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
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 leaving 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 receiving port. After receiving the IGMP group-specific query,
the switch forwards it through all its router ports in the VLAN and all member ports in that
multicast group.
The switch also performs the following actions on the port that receives the IGMP leaving
message: If the port receives any IGMP membership report in response to the