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C
HAPTER
3: U
SING
M
ULTICAST
F
ILTERING
command will configure the Switch 4200 Series to automatically
negotiate with compatible devices on VLAN 1 to become the querier.
The Switch 4200 Series is compatible with any device that conforms to
the IGMP v2 protocol.
IGMP Multicast
Filtering
IGMP is the system that all IP-supporting network devices use to register
endstations with multicast groups. It can be used on all LANs and VLANs
that contain a multicast capable IP router and on other network devices
that support IP.
IGMP multicast filtering works as follows:
1
The IP router (or querier) periodically sends
query
packets to all the
endstations in the LANs or VLANs that are connected to it.
If your network has more than one IP router, then the one with the lowest
IP address becomes the querier. The Switch can be the IGMP querier and
will become so if its own IP address is lower than that of any other IGMP
queriers connected to the LAN or VLAN. However, as the Switch only has
an IP address on its default VLAN, the Switch will only ever query on the
default VLAN (VLAN1). Therefore, if there are no other queriers on other
VLANs, the IP multicast traffic will not be forwarded on them.
2
When an IP endstation receives a query packet, it sends a
report
packet
back that identifies the multicast group that the endstation would like to
join.
3
When the report packet arrives at a port on a Switch with
IGMP multicast
learning
enabled, the Switch learns that the port is to forward traffic for
the multicast group and then forwards the packet to the router.
4
When the router receives the report packet, it registers that the LAN or
VLAN requires traffic for the multicast groups.
5
When the router forwards traffic for the multicast group to the LAN or
VLAN, the Switch units only forward the traffic to ports that received a
report packet.
Enabling IGMP Multicast Learning
You can enable or disable multicast learning and IGMP querying using the
snoopMode
command on the CLI or the web interface. For more
information about enabling IGMP multicast learning, please refer to the
Management Interface Reference Guide supplied on your Switch
CD-ROM.