30 CHAPTER 3: USING MULTICAST FILTERING
command will configure the Switch 4200 Series to automatically
negotiate with compatible devices on VLA N 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 dev ices use to register
endstations with multicast groups. It can be used on all L ANs and VLANs
that contain a multicast capable IP router and o n other network devices
that support IP.
IGMP multicast filtering works as follows:
1The 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 a ny 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 on ly ever query on the
default VLAN (VLAN1). Therefore, if there are no other que riers on other
VLANs, the IP mul ticast traffic will not b e forwarded on them.
2When 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.
3When 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.
4When the router receives the report packet, it registers that the LAN or
VLAN requires traffic fo r the multicast group s.
5When the router forwards traffic for the multicast group to the LAN or
VLAN, the Switch unit s only forward the traffic to ports th at 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 w eb interface. For more
information about enabling IGMP multicast l earning, please refer to the
Management Interface Reference Guide supplied on your Switch
CD-ROM.