How MLD snooping works

The ports in this section are dynamic ports. For information about how to configure and remove static ports, see "Configuring a static member port" and "Configuring a static router port."

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 devices 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:

MLD report

A host sends an MLD report to the MLD querier for the following purposes:

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:


[NOTE: ]

NOTE:

A Layer 2 device does not forward an MLD report through a non-router port because of the host MLD report suppression mechanism. If a non-router port has member host attached, the member hosts suppress their MLD reports upon receiving MLD reports forwarded by the non-router port. The Layer 2 device cannot know the existence of the member hosts attached to the non-router port. For more information about the MLD report suppression mechanism, see "Configuring MLD."


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.

After receiving the MLD done message on a port, the MLD querier resolves the IPv6 multicast group address in the message. Then, it sends an MLD multicast-address-specific query to the IPv6 multicast group through the receiving port.

After receiving the MLD multicast-address-specific query, the Layer 2 device forwards the query through all router ports and member ports of the group in the VLAN or VSI. Then, it waits for the responding MLD report from the directly connected hosts. For the dynamic member port that received the done message, the Layer 2 device also performs one of the following actions: