Multicast filtering
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7-7
7.4
Dynamic multicast groups
7.4.1
Internet Group Management Protocol (IGMP)
IGMP on Layer 3
The Internet Group Management Protocol describes a method for distributing information
via multicast applications between routers and termination devices at IP level (Layer 3).
When starting a multicast application, a network device transmits an IGMP membership
report and thus informs its members of a specific multicast group. A router collects these
membership reports and thus maintains the multicast groups of its subnetwork.
Query
At regular intervals, the router sends IGMP queries. This causes the devices with multicast
receiver applications to send 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 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
in the network, they determine among themselves which router performs 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 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. Please note that the MMS/MCS only
operates as the IGMP querier in the management VLAN.
IGMP snooping
A switch, which connects a multicast receiver with a router, can read and evaluate IGMP
information using the IGMP snooping method. IGMP snooping translates IP multicast group
addresses 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
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 compatible termination 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).
NOTE:
If IGMP Snooping is activated the connection to other switches is only by the use
of ports 1-8 possible.
The "IGMP Query" function only transmits in the management VLAN and only stops if
there is a better querier in the management VLAN.
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