Multicasts - Hirschmann Power MICE Reference Manual

Switching

5.4 Multicasts

5.4 Multicasts
The Internet Group Management Protocol (IGMP) describes the distribution
of Multicast information between routers and terminal devices on Layer 3.
Routers with an active IGMP function periodically send queries () to find out
which IP Multicast group members are connected to the LAN. Multicast
group members reply with a Report message. This Report message contains
all the parameters required by the IGMP. The router records the IP Multicast
group address from the Report message in its routing table. The result of this
is that it transfers frames with this IP Multicast group address in the destina-
tion address field only in accordance with the routing table.
Devices which no longer want to be members of a Multicast group can cancel
their membership by means of a Leave message (from IGMP version 2), and
they do not transmit any more Report messages. In IGMP versions 1 and 2,
the router removes the routing table entry if it does not receive any Report
messages within a specified period of time (aging time).
If there are multiple routers with an active IGMP function in the network, then
they work out among themselves which router performs the Query function.
If there is no router in the network, then a suitably equipped Switch can per-
form the Query function.
A Switch that connects a Multicast receiver with a router can evaluate the
IGMP information with the aid of the IGMP Snooping procedure.
IGMP Snooping translates IP Multicast group addresses into MAC Multicast
addresses, so that the IGMP functions can also be used by Layer 2 Switches.
The Switch enters the MAC addresses of the Multicast receivers, with are ob-
tained via IGMP Snooping from the IP addresses, in the static address table.
Thus the Switch blocks Multicast packets at the ports at which no Multicast
receivers are connected.
The GARP Multicast Registration Protocol (GMRP) describes the distribution
of data packets with a Multicast address as the destination address on Layer
2. Devices that want to receive data packets with a Multicast address as the
destination address use the GMRP to register the Multicast address. For a
Switch, registration involves entering the Multicast address in the filter table.
When a Multicast address is entered in the filter table, the Switch sends this
information in a GMRP packet to all the ports. Therefore the connected
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