How Igmp Snooping Works - HP FlexFabric 5700 Series Configuration Manual

How IGMP snooping works

The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "Configuring static
IGMP messages types include general query, IGMP report, and leave message. An IGMP
snooping-enabled switch performs differently depending on the message.
General query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local subnet to
check for the existence of multicast group members.
After receiving an IGMP general query from the IGMP querier, the switch forwards the query to all ports
in the VLAN except the receiving port. The switch also performs one of the following actions:
• If the receiving port is a dynamic router port in the router port list, the switch restarts the aging timer
for the port.
If the receiving port does not exist in the router port list, the switch adds the port to the router port
•
list. It also starts an aging timer for the port.
IGMP report
A host sends an IGMP report to the IGMP querier for the following purposes:
Responds to queries if the host is a multicast group member.
•
Applies for a multicast group membership.
•
After receiving an IGMP report from a host, the switch forwards the report through all the router ports in
the VLAN. It also resolves the address of the reported multicast group, and looks up the forwarding table:
If no match is found, the switch creates a forwarding entry with the receiving port as an outgoing
•
interface. It also marks the receiving port as a dynamic member port and starts an aging timer for
the port.
If a match is found but the receiving port is not in the forwarding entry, the switch adds the port as
•
an outgoing interface to the forwarding entry. It also marks the receiving port as a dynamic member
port and starts an aging timer for the port.
If a match is found and the receiving port is in the forwarding entry, the switch restarts the aging
•
timer for the port.
In an application with a multicast group policy configured on an IGMP snooping-enabled switch, when
a user requests a multicast program, the user's host initiates an IGMP report. After receiving this report,
the switch resolves the multicast group address in the report and performs ACL filtering on the report. If
the report passes ACL filtering, the switch creates an IGMP snooping forwarding entry with the receiving
port as an outgoing interface. Otherwise, the switch drops this report, which means the receiver does not
successfully join the multicast group and cannot retrieve the program.
A switch does not forward an IGMP report through a non-router port because of the IGMP report
suppression mechanism.
Leave message
An IGMPv1 receiver does not send any leave messages when it leaves a multicast group. The switch does
not remove the port that connects to the receiver from the outgoing port list of the associated forwarding
entry until the aging timer for the port expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message when it leaves a multicast group.
ports."
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