How Igmp Snooping Operates - HP 1910 User Manual

Timer
Dynamic member port
aging timer
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.

How IGMP snooping operates

In this section, the ports involved are dynamic ports.
An IGMP snooping–enabled switch performs different actions when it receives different IGMP messages.
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 check whether any active multicast group members exist on the subnet.
After receiving an IGMP general query, the switch forwards it through all ports in the VLAN (except the
port that received the query). The switch performs the following judgment:
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.
When receiving a membership report
A host sends an IGMP membership report to the IGMP querier in the following circumstances:
After receiving an IGMP query, a multicast group member host responds with an IGMP report.
•
• When the host wants to join a multicast group, it sends an IGMP report to the querier to announce
its interest in the multicast information addressed to that group.
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. The switch also performs the following judgment:
• 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 that 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 that 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 resets an aging timer for that port.
A switch does not forward an IGMP report through a non-router port. The reason is that if the switch
forwards a report through a member port, all the attached hosts that monitor the reported multicast group
address, according to the IGMP report suppression mechanism, suppress their own reports after
Description
When a port dynamically
joins a multicast group,
the switch sets an aging
timer for the port. When
the timer expires, the
dynamic member port
ages out.
230
Message before expiry
IGMP membership report
Action after expiry
The switch removes this
port from the IGMP
snooping forwarding
table.