How Igmp Snooping Works - HP 1920 Gigabit Ethernet Switch Series User Manual

Timer
Dynamic
member port
aging timer
NOTE:
In IGMP snooping, only dynamic ports age out.

How IGMP snooping works

The ports in this section are dynamic ports.
IGMP messages 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
determine whether any active multicast group members exist on the subnet. The destination address of
IGMP general queries is 224.0.0.1.
After receiving an IGMP general query, the switch forwards the query through 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 is not in the router port list, the switch adds the port as a dynamic router port into
•
the router port list and 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 IGMP 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 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.
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 running on hosts causes all
attached hosts that monitor the reported multicast address to suppress their own reports. In this case, the
Description
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.
Message received before
the timer expires
IGMP membership report.
254
Action after the timer
expires
The switch removes this
port from the IGMP
snooping forwarding
table.