31.3.2 IP addressing control method
The swrt_multicast_table command can be set on the Switch to use both IPv4 multicast and IGMP snooping at the same time on the same VLAN. When using IPv4 multicast and IGMP snooping at the same time, make sure that you use IPv4 multicasts on the corresponding VLAN.
- <Structure of this section>
(1) IP address learning
Multicast IP addresses are learned dynamically when IGMP messages are received on VLANs for which IGMP snooping is set. Information about learned multicast IP addresses is set in multicast forwarding entries for IPv4 multicast.
(a) Registering Entries
When an IGMPv1or IGMPv2 Report message or IGMPv3 Report (membership request) message is received, the multicast IP address is learned from the multicast group address included in the message, and an entry is created that forwards traffic bound for the multicast group only to ports for which IGMPv1, IGMPv2, or IGMPv3 Report messages were received.
(b) Deleting an entry
Learned multicast IP addresses are deleted in any of the following cases when group members no longer exist on all ports:
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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 the Switch is the representative querier). 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.
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An IGMPv3 Report (leave request) message is received.
When an IGMPv3 Report (leave request) message whose multicast address record type is CHANGE_TO_INCLUDE_MODE is received, Group-Specific Query messages are sent from the Switch to the port that received the message, twice every second (Group-Specific Query messages are only sent when the Switch is the representative querier). 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 an IGMPv3 Report message whose multicast address record type is BLOCK_OLD_SOURCES is received, Group-and-Source-Specific Query messages are sent from the Switch, twice every second (Group-and-Source-Specific Query messages are only sent when the Switch is the representative querier). Entries are deleted on timeout regardless of the response to Group-Source-and-Specific Query messages.
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.
- Notes
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The timeout period is Group Membership Interval period.
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A set time elapses after an IGMPv1, IGMPv2, or IGMPv3 Report (membership request) message is received.
Multicast routers periodically send Query messaging to verify that group members exist on 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
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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
(2) Layer 2 forwarding of IPv4 multicast packets
Layer 2 forwarding within VLANs receiving IPv4 multicast packets is performed based on IP address. Layer 2 forwarding based on IGMP snooping results is performed for all ports that receive IGMP Report (membership request) messages.
(3) Layer 3 forwarding of IPv4 multicast packets
When Layer 3 forwarding based on IPv4 multicast is performed between VLANs, and IGMP snooping is running on the forwarding destination VLAN, multicast traffic for which Layer 3 forwarding is performed is forwarded according to the learning results for IGMP snooping within the forwarding destination VLAN.
(4) Specific Query Transmission with IPv4 Multicast Concurrent Use
IPv4 multicast can be run so that when the Switch is the representative querier in the VLAN, Group-Specific Queries or Group-and-Source-Specific Queries sent due to IGMP Leave message or IGMPv3 Report (leave request) message reception are sent to all ports in the VLAN and not just the recipient port.