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Multicast filtering
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When starting a multicast application, a network device transmits an IGMP membership re-
port and thus announces its membership of a specific multicast group. A router collects
these membership reports, maintaining in this way the multicast groups of its subnetwork.
Query At regular intervals, the router sends IGMP queries. This prompts the devices with multicast
receiver applications to send another membership report.
The router enters the IP multicast group address from the report message in its routing ta-
ble. This means that frames with this IP multicast group address in the destination address
field are only transferred according to the routing table. Devices that are no longer members
of a multicast group log out with a leave message (IGMP Version 2 or later) and no longer
send report messages.
The router also removes the routing table entry if it does not receive a report message within
a specific time (aging time). If several routers with active IGMP query function are connected
to the network, they determine among themselves which router performs the query function.
This depends on the IP addresses, as the router with the lowest IP address continues to op-
erate as the querier and all the other routers no longer send query messages. If these rout-
ers do not receive a new query telegram within a specific period of time, they themselves
become queriers again. If there are no routers in the network, a suitably equipped switch
can be used for the query function. Please note that the MMS/MCS only operates as the
IGMP querier in the management VLAN.
IGMP snooping A switch that connects a multicast receiver with a router can read and evaluate IGMP infor-
mation using the IGMP snooping method. IGMP snooping translates IP multicast group ad-
dresses into multicast MAC addresses, so that the IGMP function can also be detected by
Layer 2 switches. The switch enters the MAC addresses of the multicast receivers, which
were obtained from the IP addresses by IGMP snooping, in its own multicast filter table.
Thus the switch filters multicast packets of known multicast groups and only forwards pack-
ets to those ports to which corresponding multicast receivers are connected.
IGMP snooping can only be used on Layer 2 if all terminal devices send IGMP messages.
The IP stack of multicast-compatible terminal devices with applications linked to a multicast
address automatically sends the relevant membership reports.
IGMP snooping operates independently of the Internet Group Management Protocol
(IGMP).
The “IGMP Query” function only transmits in the management VLAN and only stops if
there is a better querier in the management VLAN.