31.3.1 MAC addressing control method

(a) 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 31-3: Correspondence between IPv4 Multicast Address and MAC Address

(b) 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 IGMPv2 Leave message is received.

  Group-Specific Query messages are sent from the Switch to the port that received IGMPv2 Leave 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.

  When IGMP instant leave is used and an IGMPv2 Leave message is received, the corresponding port is instantly deleted from the entries. Even when a querier is set, Group-Specific Query messages are not sent.

• An IGMPv3 Report (leave request) message is received.

  Group-Specific Query messages are sent from the Switch to the port that received IGMPv3 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 IGMPv3 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.

  When IGMP instant leave is used, and an IGMPv3 Report (leave request) message whose multicast address record type is CHANGE_TO_INCLUDE_MODE is received, the corresponding port is immediately deleted from the entries. Even when a querier is set, Group-Specific Query messages are not sent.

• A set time elapses after an IGMPv1, IGMPv2, or IGMPv3 Report (membership request) message is received.

  Multicast routers regularly send Query messages to check that group members exist in directly connected interfaces. When the Switch receives an IGMP Query message from a router, it forwards it to all ports in the VLAN. If there is no response to the IGMP 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 Group Membership Interval times or IGMPv1/IGMPv2/IGMPv3 Report messages, the corresponding entries are deleted.

  Notes

  Group Membership 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 IGMPv3 with a representative querier

  RV,QI = retrieve from received Query messaging

  QRI=10 seconds

  .When the local device is operating in IGMPv3 with the representative querier or in IGMPv2

  RV=2

  QI=125 seconds

  QRI=10 seconds