How Igmp Snooping Works - HP 6125XLG Ip Multicast Configuration Manual

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

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
To check for the existence of multicast group members, the IGMP querier periodically sends IGMP
general queries to all hosts and routers on the local subnet. All these hosts and routers are identified by
the address 224.0.0.1.
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. 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 the host, the switch forwards it through all the router ports in the
VLAN. It also resolves the address of the reported multicast group, and looks up the forwarding table for
a matching entry:
If no match is found, the switch creates a forwarding entry and adds the receiving 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 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 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 performs an ACL filtering on the report. If the
report passes the ACL filtering, the switch creates an IGMP snooping forwarding entry with the receiving
port as an outgoing interface. 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 because of the IGMP report
suppression mechanism. Assuming the switch forwards a report message through a member port, the
attached member receivers will receive the report and suppress their own reports. In this case, the switch
ports."
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