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24.3 IGMP Snooping
The following explains IGMP snooping functionality and its operation. The format and
timers for IGMP messages sent and received by the Switch conform to RFC 2236.
When IGMP snooping is used, the MAC address control method is used to control
forwarding for multicast traffic.
24.3.1 MAC address control method
(1) MAC address learning
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.
(a) Registering entries
When an IGMP Report 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 IGMP 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 24-3 Correspondence between IPv4 multicast addresses and MAC addresses
(b) Deleting entries
Learned multicast MAC addresses are deleted in either of the following cases when group
members no longer exist on all ports:
An IGMP Leave message is received.
Group-Specific Query messages are sent from the Switch to the port that received
IGMP 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.
A set time elapses after an IGMP 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
24 Description of IGMP Snooping and MLD Snooping
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