Timer

Description

Message received before

Action after the timer

the timer expires

expires

 

 

Dynamic member port aging timer

When a port dynamically

 

 

joins a multicast group, the

 

The switch removes this

switch starts or resets an aging

IGMP membership report.

port from the IGMP

timer for the port. When the

snooping forwarding

 

timer expires, the dynamic

 

table.

member port ages out.

 

 

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

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