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How MLD snooping works
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "
MLD messages include general query, MLD report, and done message. An MLD snooping-enabled
Layer 2 device performs differently depending on the MLD message types.
General query
The MLD querier periodically sends MLD general queries to all hosts and routers on the local subnet
to check for the existence of IPv6 multicast group members.
After receiving an MLD 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:
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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 router 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.
MLD report
A host sends an MLD report to the MLD querier for the following purposes:
•
Responds to queries if the host is an IPv6 multicast group member.
•
Applies for an IPv6 multicast group membership.
After receiving an MLD 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 IPv6 address of the reported IPv6 multicast
group, and looks up the forwarding table for a matching entry as follows:
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If no match is found, the Layer 2 device creates a forwarding entry for the group with the
receiving port 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 port as a
dynamic member port to the forwarding entry 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 an IPv6 multicast group policy configured on an MLD snooping-enabled Layer
2 device, when a user requests a multicast program, the user's host initiates an MLD report. After
receiving this report message, the Layer 2 device resolves the IPv6 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 MLD snooping forwarding entry for the group with the receiving port as an outgoing
interface. If the report does not pass ACL filtering, the Layer 2 device drops this report message, in
which case, the IPv6 multicast data for the IPv6 multicast group is not sent to this port, and the user
cannot retrieve the program.
A Layer 2 device does not forward an MLD report through a non-router port because of the host MLD
report suppression mechanism.
Done message
When a host leaves an IPv6 multicast group, the host sends an MLD done message to the Layer 3
devices. When the Layer 2 device receives the MLD done message on a dynamic member port, the
Layer 2 device first examines whether a forwarding entry matches the IPv6 multicast group address
in the message.
•
If no match is found, the Layer 2 device discards the MLD done message.
•
If a match is found but the receiving port is not an outgoing interface in the forwarding entry, the
Layer 2 device discards the MLD done message.
