User manual

Multicast Filtering
7278_en_01 PHOENIX CONTACT 5-1
5 Multicast Filtering
5.1 Basics
Multicast Multicast applications - other than Unicast applications using point-to-point communication
- do not transmit their data using a MAC address of the target, but they use a multicast
group address that is target-independent. Always using wireless communication, a station
transmits one data packet that is received by one or more receiving stations.
Advantages:
1 In case one data packet of a transmitter is to be transmitted to 8 receivers, the same
packet must not be sent 8 times to the addresses of all 8 devices. Instead it only needs
to be sent once to the address of the multicast group that includes 8 devices.
2 When using multicast communication and filtering, the required bandwidth for data
transmission is reduced because every packet can only be transmitted once.
5.2 Dynamic Multicast Groups
5.2.1 Internet Group Management Protocol - IGMP
IGMP on Layer 3 The Internet Group Management Protocol describes a procedure for distributing
information via mulitcast applications between routers and termination devices on the IP
level (layer 3).
When starting a multicast application, a network device transmits a so-called IGMP
membership report and thus publishes its membership of a specified multicast group. A
router collects these membership reports and thus maintains the multicast groups of its sub
networks.
Query At regular intervals, the router transmits IGMP queries. This causes the devices with
multicast-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 determine 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.
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 MAC multicast addresses, so that the IGMP function can also be