3 description of igmp snooping – Pilz PSSnet SHL 8T MRP User Manual
Page 108
Network load control
PSSnet SHL - Basic Configuration
Pilz GmbH & Co. KG, Felix- Wankel Str. 2, 73760 Ostfildern
8.2 Multicast application
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8.2.3 Description of IGMP Snooping
The Internet Group Management Protocol (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 destina-
tion 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 (in IGMP version 2) which router car-
ries out the Query function. If there is no router in the network, then a suitably
equipped switch can perform 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, with are
obtained 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.