Understanding Igmp Snooping; Receiving A General Query; Receiving A Membership Report - HP A6600 Configuration Manual

Understanding IGMP snooping

A switch running IGMP snooping performs different actions when it receives different IGMP messages.
The description about adding or deleting a port in this section is only for a dynamic port. Static ports can
be added or deleted only through the corresponding configurations.

Receiving a general query

The IGMP querier periodically sends IGMP general queries to all hosts and routers (224.0.0.1) on the
local subnet to determine whether 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
receiving port and performs the following to the receiving port:
If the receiving port is a dynamic router port existing in its router port list, the switch resets the aging
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timer of this dynamic router port.
If the receiving port is not a dynamic router port existing in its router port list, the switch adds it into
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its router port list and sets an aging timer for this dynamic router port.

Receiving a membership report

A host sends an IGMP report to the IGMP querier in the following circumstances:
• After receiving an IGMP query, a multicast group member host responds with an IGMP report.
When intended to join a multicast group, a host sends an IGMP report to the IGMP querier to
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announce that it is interested in the multicast information addressed to that group.
After receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group, and performs the following operation:
If no forwarding table entry exists for the reported group, the switch creates an entry, adds the port
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as a dynamic member port to the outgoing port list, and starts a member port aging timer for that
port.
If a forwarding table entry exists for the reported group, but the port is not included in the outgoing
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port list for that group, the switch adds the port as a dynamic member port to the outgoing port list,
and starts an aging timer for that port.
If a forwarding table entry exists for the reported group and the port is included in the outgoing port
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list, which means that this port is already a dynamic member port, the switch resets the aging timer
for that port.
A switch does not forward an IGMP report through a non-router port. This is because if the switch
forwards a report message through a member port, all the attached hosts listening to the reported
multicast address will suppress their own reports after receiving this report according to the IGMP report
suppression mechanism on them, and this will prevent the switch from knowing whether the reported
multicast group still has active members attached to that port. For more information about the IGMP report
suppression mechanism on hosts, see "Configuring IGMP."
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