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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.
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
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 router port list, the Layer 2 device restarts the
aging timer for the router port.
•
If the receiving port does not exist in the router port list, the Layer 2 device adds the port to the
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. It also resolves the IPv6 address of the reported IPv6 multicast group, and
looks up the forwarding table for a matching entry:
•
If no match is found, the Layer 2 device creates a forwarding entry for the group and adds the
receiving port as an outgoing interface to the forwarding entry. 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 receiving port is not in the forwarding entry, the Layer 2 device adds
the receiving port as an outgoing interface to the forwarding entry. 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 receiving port is in the forwarding entry, 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 IPv6 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
message, which means the receiver does not successfully join the IPv6 multicast group and cannot
retrieve the program.
A Layer 2 device does not forward an MLD report through a non-router port because of the MLD
report suppression mechanism.
Done message
When a host leaves an IPv6 multicast group, the host sends an MLD done message to the multicast
routers. 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 in the forwarding entry, the Layer 2 device
discards the MLD done message.
