How Igmp Snooping Works - HP 5920 series Configuration Manual

NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.

How IGMP snooping works

An IGMP snooping-enabled switch 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
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers identified by the
address 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 switch forwards the query to all ports in the VLAN except the
port that received the query. 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, 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:
Responds to queries if the host is a multicast group member.
•
Applies for a multicast group membership.
•
After receiving an IGMP report from the host, the switch forwards it through all the router ports in the
VLAN, resolves the address of the reported multicast group, and performs one of the following actions:
If no forwarding entry matches the group address, the switch creates a forwarding entry for the
•
group, adds the receiving 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, but the receiving port is not in the forwarding
•
entry for the group, the switch 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 receiving port is in the forwarding entry
•
for the group, the switch restarts the aging timer for the port.
In an application with a group filter 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 looks up the ACL. If a match is found to
permit the port that received the report to join the multicast group, the switch creates an IGMP snooping
forwarding entry for the multicast group and adds the port to the forwarding entry. Otherwise, the switch
drops this report, in which case the multicast data for the multicast group is not sent to this port, and the
user cannot retrieve the program.
A switch does not forward an IGMP report through a non-router port. If the switch 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. In this case, the switch cannot
determine whether the reported multicast group still has active members attached to that port. For more
information about the IGMP report suppression mechanism, see
ports."
15
"Configuring IGMP."