Wavetronix Click 342 (lean managed ethernet switch) (CLK-342) - Managed Switches User Guide - Manual (Page 69)

68 CHAPTER 7 • MULTICAST FILTERING
At regular intervals, the router transmits IGMP queries. This causes the devices with multi-
cast-receiver applications to transmit a membership report again.
The router enters the IP multicast group address from the report message in its routing
table. This means that frames with this IP multicast group address in the target 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
any 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 in the network, they de-
termine amongst themselves which router carries out the query function. This depends on
the IP address, as the router with the lowest IP address continues to operate as the querier
and all the other routers no longer send any query messages. If these routers 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.
A switch, which connects a multicast receiver with a router, can read and evaluate IGMP in-
formation using the IGMP snooping method. IGMP snooping translates IP multicast group
addresses into MAC multicast 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, into its own multicast table. Thus
the switch filters multicast packets of known multicast groups and only forwards packets to
those ports to which corresponding multicast receivers are connected.
IGMP snooping can only be used on layer 2 if all termination devices send IGMP messages.
The IP stack of multicast termination devices with applications linked to a multicast address
automatically sends the relevant membership reports.
Note
IGMP snooping operates independently of IGMP.
When IGMP snooping is enabled, if the switch receives multicast data for which no IGMP
reports have been received and for which no multicast group currently exists, the switch
creates the group automatically. This multicast data is then forwarded to the querier, i.e.
the querier port is added by default to the group. If the switch itself is the active querier,
then these multicasts are blocked on all ports. Any subsequent IGMP reports for this group
received on other ports are handled normally, that is, the port is added as a member to the
multicast group.