Configuration Guide Vol. 1

32.3.1 Learning MAC Addressing

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

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(1) Registering Entries

When an IGMPv1 or IGMPv2 Report message or IGMPv3 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 IGMPv1, IGMPv2, or IGMPv3 Report messages have been received.

Destination MAC addresses for IPv4 multicast data are generated by copying the lowest 23 bits of the IP address to the MAC address. Therefore, MAC addresses will be redundant for IP addresses for which the lower 23 bits are the same. For example, the multicast MAC address for both 224.10.10.10 and 225.10.10.10 is 0100.5E0A.0A0A. These addresses are treated as packets bound for the same MAC address by Layer 2 forwarding. The following figure shows the correspondence between IPv4 multicast addresses and MAC addresses.

Figure 32-3 Correspondence between IPv4 multicast addresses and MAC addresses

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(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:

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