Hirschmann MACH 4000 Series, User Manual

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Traffic control
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8.2 Multicast application
Basic - L3P
Release 3.1 06/07
8.2.3 Description IGMP snooping
The I nternet G roup M anagement P rotocol (IGMP) describes the distribution
of Multicast information between routers and terminal devices on the Layer 3
level.
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 target
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 a number of routers with an active IGMP function in the network,
then they work out among themselves which router carries out the Query
function when using IGMP version 2. If there is no router in the network, then
a suitably equipped Switch can carry out 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 records the MAC addresses of the Multicast receivers, which are
obtained by the 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.