H3C Technologies H3C S12500 Series Switches - Manual (Page 31)

15
Timer Description
Message before
expiration
Action after expiration
Dynamic member port
aging timer
When a port dynamically
joins a multicast group,
the switch starts or resets
an aging timer for the
port. When the timer
expires, the dynamic
member port ages out.
IGMP membership
report.
The switch removes this
port from the IGMP
snooping forwarding
table.
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
How IGMP snooping operates
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "
Configuring static ports ."
IGMP messages types are as follows: 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 (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 it to all ports in the VLAN, except the port
that received the query. The switch also performs one of the following actions:
• If the port that received the query is a dynamic router port in the router port list of the switch, the
switch restarts the aging timer for the port.
• If the port that received the query is not in the router port list of the switch, the switch adds it into the
router port list as a dynamic router port and starts an aging timer for the port.
IGMP report
A host sends an IGMP report to the IGMP querier in the following circumstances:
• Responds to queries if the host is a multicast group member.
• Applies for a multicast group membership.
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 one of the following actions:
• If no forwarding entry matches the group address, the switch 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 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 port that received the IGMP report is in the
forwarding entry for the group, the switch restarts the aging timer for the 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