R3102-R3103-HP 6600/HSR6600 Routers IP Multicast Configuration Guide

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NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
How IGMP snooping works
An IGMP snooping-enabled router performs different actions when it receives different IGMP messages.
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "Configuring static ports."
When 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 any active multicast group members exist on the subnet.
After receiving an IGMP general query, the router forwards it to all ports in the VLAN, except the port that
received the query. The router also performs one of the following actions:
• If the receiving port is a dynamic router port in the router port list, restarts the aging timer for the
port.
• If the receiving port is not in the router port list, adds it into the router port list as a dynamic router
port and starts an aging timer for the port.
When receiving a membership report
A host sends an IGMP report to the IGMP querier for the following purposes:
• If the host has been a member of a multicast group, responds to an IGMP query.
• Applies for joining a multicast group.
After receiving an IGMP report, the router forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group, and performs the following judgment:
• If no forwarding entry matches the group address, the router creates a forwarding entry for the
group, adds the port that received the IGMP report as a dynamic member port to the forwarding
entry, and starts an aging timer for the port.
• If a forwarding entry matches the group address, but the port that received the IGMP report is not
in the forwarding entry for the group, the router adds the port as a dynamic member port to the
forwarding entry, and starts an aging timer for the port.
• If a forwarding entry matches the group address and the port that received the IGMP report is in the
forwarding entry for the group, the router restarts the aging timer for the port.
A router does not forward an IGMP report through a non-router port. If the router forwards a report
message through a member port, the IGMP report suppression mechanism causes all attached hosts that
monitor the reported multicast address to suppress their own reports. This makes the router unable to
know whether the reported multicast group still has active members attached to that port. For more
information about the IGMP report suppression mechanism, see "Configuring IGMP."
When receiving a leave message
An IGMPv1 host silently leaves a multicast group and the router is not notified of the leave. However,
because the host stops sending IGMP reports as soon as it leaves the multicast group, the router removes
the port that connects to the host from the forwarding entry for the multicast group when the aging timer
for the port expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when leaving a
multicast group.