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.

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:

• 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.

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:

• 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.

	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."

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.

• If a match is found and the receiving port is not the only outgoing interface in the forwarding entry, the Layer 2 device performs the following actions:

  • Discards the MLD done message.

  • Sends an MLD multicast-address-specific query to identify whether the group has active listeners attached to the receiving port.

  • Sets the aging timer for the receiving port to twice the MLD last listener query interval.

• If a match is found and the receiving port is the only outgoing interface in the forwarding entry, the Layer 2 device performs the following actions:

  • Forwards the MLD done message to all router ports in the VLAN or VSI.

  • Sends an MLD multicast-address-specific query to identify whether the group has active listeners attached to the receiving port.

  • Sets the aging timer for the receiving port to twice the MLD last listener query interval.

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:

• If the port receives an MLD report before the aging timer expires, the Layer 2 device resets the aging timer for the port.

• If the port does not receive any MLD report messages when the aging timer expires, the Layer 2 device removes the port from the forwarding entry for the IPv6 multicast group.