User Manual
UMN:CLI
V8102
489
10.2.2
IGMP Snooping Basic
Layer 2 switches normally flood multicast traffic within the broadcast domain, since it has
no entry in the Layer 2 forwarding table for the destination address. Multicast addresses
never appear as source addresses, therefore the switch cannot dynamically learn mul-
ticast addresses. This multicast flooding causes unnecessary bandwidth usage and dis-
carding unwanted frames on those nodes which did not want to receive the multicast
transmission. To avoid such flooding, IGMP snooping feature has been developed.
The purpose of IGMP snooping is to constrain the flooding of multicast traffic at Layer 2.
IGMP snooping, as implied by the name, allows a switch to snoop the IGMP transaction
between hosts and routers, and maintains the multicast forwarding table which contains
the information acquired by the snooping. When the switch receives a join request from a
host for a particular multicast group, the switch then adds a port number connected to the
host and a destination multicast group to the forwarding table entry; when the switch re-
ceives a leave message from a host, it removes the entry from the table.
By maintaining this multicast forwarding table, the V8102 dynamically forward multicast
traffic only to those interfaces that want to receive it as nominal unicast forwarding does.
Fig. 10.4
IGMP Snooping
