15
How IGMP snooping works
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "
."
IGMP messages types include general query, IGMP report, and leave message. An IGMP
snooping-enabled Layer 2 device performs differently depending on the message types.
General query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local
subnet to check for the existence of multicast group members.
After receiving an IGMP general query, the Layer 2 device forwards the query to all ports in the VLAN
or VSI except the receiving port. The Layer 2 device also performs one of the following actions:
•
If the receiving port is a dynamic router port in the dynamic router port list, the Layer 2 device
restarts the aging timer for the port.
•
If the receiving port does not exist in the dynamic router port list, the Layer 2 device adds the
port to the dynamic 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 a host, the Layer 2 device forwards the report through all the
router ports in the VLAN or VSI. It also resolves the address of the reported multicast group, and
looks up the forwarding table for a matching entry as follows:
•
If no match is found, the Layer 2 device creates a forwarding entry with the receiving port as an
outgoing interface. 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 matching forwarding entry does not contain the receiving port, the
Layer 2 device adds the receiving port to the outgoing interface list. 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 matching forwarding entry contains the receiving port, the Layer 2
device restarts the aging timer for the port.
In an application with a group policy configured on an IGMP snooping-enabled Layer 2 device, when
a user requests a multicast program, the user's host initiates an IGMP report. After receiving this
report, the Layer 2 device resolves the multicast group address in the report and performs ACL
filtering on the report. If the report passes ACL filtering, the Layer 2 device creates an IGMP
snooping forwarding entry for the multicast group with the receiving port as an outgoing interface. If
the report does not pass ACL filtering, the Layer 2 device drops this report. The multicast data for the
multicast group is not sent to this port, and the user cannot retrieve the program.
A Layer 2 device does not forward an IGMP report through a non-router port because of the host
IGMP report suppression mechanism. For more information about the IGMP report suppression
mechanism, see "Configuring IGMP."
Leave message
An IGMPv1 receiver host does not send any leave messages when it leaves a multicast group. The
Layer 2 device cannot immediately update the status of the port that connects to the receiver host.
The Layer 2 device does not remove the port from the outgoing interface list in the associated
forwarding entry until the aging time for the group expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message when it leaves a multicast group.
When the Layer 2 device receives an IGMP leave message on a dynamic member port, the Layer 2
device first examines whether a forwarding entry matches the group address in the message.
