Multicast filtering
7278_en_04 PHOENIX CONTACT 5-1
5 Multicast filtering
5.1 Basics
Multicast Multicast applications, unlike unicast applications with point-to-point communication, do not
transmit their data with the MAC address of the destination, but with an independent
multicast group address. Always using connectionless communication, a station transmits
one data packet that is received by one or more receiver stations.
Advantages:
1 If, for example, a data packet of a transmitter is to be transmitted to eight receivers, the
same packet does not have to be sent eight times to the addresses of all eight devices.
Instead it only needs to be sent once to the address of the multicast group that includes
the eight devices.
2 When using multicast communication and filtering, the required bandwidth for data
transmission is reduced because each 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 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.
To Next Page
Loading...
