30 CHAPTER 3: USING MULTICAST FILTERING
You would enable query mode if you wish to run multicast sessions in a
network that does not contain any IGMP routers (or queriers). This
command will configure the Switch to automatically negotiate with
compatible devices on VLAN 1 to become the querier.
The Switch 3812 and Switch 3824 are compatible with any device that
conforms to the IGMP v2 protocol. The Switch does not support IGMP v3.
If you have an IGMP v3 network, you should disable IGMP snooping for
the Switch using the snoopMode command on the Web Interface .
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.
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