32.4.1 Learning MAC Addressing

For VLANs for which MLD snooping is set, multicast MAC addresses are dynamically learned when MLD messages are received. The learned multicast MAC addresses are registered to the MAC address table.

<Structure of this section>

(1) Registering Entries

When an MLDv1 Report message and or MLDv2 Report (membership request) message is received, the multicast MAC address is learned from the multicast group address included in the message, and an entry is created that forwards traffic bound for a multicast group only to ports for which MLDv1 or MLDv2 Report messages have been received. Destination MAC addresses for IPv6 multicast data are generated by copying the lowest 32 bits of the IP address to the MAC address.

IPv6 multicast addresses have two types of formats for group ID fields that identify multicast groups: a 112-bit format and a 32-bit format. When group ID fields use the 112-bit address format, duplicate MAC addresses occur the same as for IPv4 multicast addresses. The following figure shows the correspondence between IPv6 multicast addresses and MAC addresses.

Figure 32-4 Correspondence between IPv6 multicast addresses and MAC addresses

(2) Deleting an entry

Learned multicast MAC addresses are deleted in any of the following cases when group members no longer exist on all ports:

An MLDv1 Done message is received.

Group-Specific Query messages are sent from the Switch to the port that received MLDv1 Done messages, twice every second (Group-Specific Query messages are only sent when a querier is set and are sent from a representative querier otherwise). If there is no response, only that port is deleted from the entries (forwarding of multicast traffic to this port is suppressed). If no group members are left in all ports in the VLAN, the entry itself is deleted.
An MLDv2 Report (leave request) message is received.

Group-Specific Query messages are sent from the Switch to the port that received MLDv2 Report (detachment request) messages, twice every second (Group-Specific Query messages are only sent when a querier is set and are sent from a representative querier otherwise). If there is no response, only that port is deleted from the entries (forwarding of multicast traffic to this port is suppressed). If no group members are left in all ports in the VLAN, the entry itself is deleted. However, when an MLDv2 Report message whose multicast address record type is BLOCK_OLD_SOURCES is received, Group-Specific Query messages are sent and entry deletion processing is performed only when a querier has been set for the local device.
A set time elapses after an MLDv1 or MLDv2 Report (membership request) message is received.

Multicast routers regularly send MLD Query messages to check that group members exist in directly connected interfaces. When the Switch receives an MLD Query message from a router, it forwards it to all ports in the VLAN. If there is no response to the MLD Query message, only that port is deleted from the entries. When no response is received from any port, the entry itself is deleted.

If the Switch does not receive Multicast Listener Interval times or MLDv1/MLDv2 Report messages, the corresponding entries are deleted.

Notes

Multicast Listener Interval is calculated in Robustness Variable(RV)NGQuery Interval(QI)+Query Response Interval(QRI).

RV,QI,QRI is shown below.

.When another device is operating in MLDv2 with a representative querier

RV,QI = retrieve from received Query messaging

QRI=10 seconds

.When the Switch operates in MLDv2 with a representative querier or in MLDv1

RV=2

QI=125 seconds

QRI=10 seconds