User manual

FL SWITCH LM
5-2
PHOENIX CONTACT 7278_en_01
detected by layer-2 switches. The switch enters the MAC addresses of the multicast
receivers, which were obtained from the IP addresses by IGMP snooping, into its own
multicast table. Thus the switch filters multicast packets of known multicast groups and
only forwards packets to those ports to which corresponding multicast receivers are
connected.
IGMP snooping can only be used on layer 2 if all termination devices send IGMP
messages. The IP stack of multicast termination devices with applications linked to a
multicast address automatically sends the relevant membership reports.
IGMP snooping operates independently of the Internet group management protocol
(IGMP).
5.2.1.1 Extended Multicast Filtering
When IGMP Snooping is enabled, if the switch receives multicast data for which no IGMP
reports have been received and for which no multicast group currently exists, the switch
creates the group automatically. This multicast data is then forwarded to the querier, i.e.
the querier port is added by default to the group. If the switch itself is the active querier,
then these multicasts are blocked on all ports. Any subsequent IGMP reports for this group
received on other ports are handled normally, that is, the port is added as a member to the
multicast group.
5.2.2 "General Multicast Configuration" Website
This website provides global settings for multicast support. Here, IGMP snooping can be
enabled and an aging time can be specified for IGMP snooping information.
Figure 5-1 "General Multicast Configuration" Website
IGMP Snooping
In IGMP snooping, the switch passively listens in on the IGMP messages that are sent over
the network and dynamically creates the appropriate groups. These groups are not stored
and will be lost with every power-down or when switching off the snooping function.
The switch supports 50 multicast groups at the same time.