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Chapter 14: IGMP Snooping Querier
172
Overview
Multicast routers are an essential part of IP multicasting. They send out
queries to the network nodes to determine group memberships, route the
multicast packets across networks, and maintain lists of the multicast
groups and the ports where group members are located.
IGMP snooping querier can be used in place of multicast routers in
situations where IP multicasting is restricted to a single LAN, without the
need for routing. This feature enables the switch to mimic a multicast
router by sending out general IGMP queries to the host nodes.
IGMP snooping querier supports IGMP version 1, version 2, and version
3. By default, the switch sends version 2 messages. If it receives version 1
messages from any of the nodes, the switch sends version 1 queries. If
the switch receives version 3 messages, all nodes respond with version 3
messages. By default, the interval at which the querier sends out IGMP
querier reports is 125 seconds. The switch reverts to version 2 queries if,
after 255 seconds, no additional version 1 or version 3 messages are
received.
The switch must have an IP address to add to the queries as its source
address. In addition, the address must be a member of the same network
as the host nodes and the multicasting source. You assign an IP address
to the switch by creating a routing interface in the VLAN. Then apply the IP
address to the VLAN where it sends its queries, to enable IGMP snooping
querier on the VLAN. Allied Telesis recommends using the Default VLAN
which has a VID of 1.
IGMP snooping querier must be used in conjunction with IGMP snooping.
Activate IGMP snooping on all of the switches in the LAN, including the
switches running the IGMP snooping querier. The switches use IGMP
snooping to monitor the responses of the host nodes to the general IGMP
queries sent by the IGMP snooping querier. From the responses, they
create lists of ports that have host nodes that want to join the various
multicast groups and forward the multicast packets to only those ports.
Figure 61 on page 173 provides an example of IGMP snooping querier on
a LAN. It consists of a single switch with one VLAN, the Default VLAN.
Both IGMP snooping and IGMP snooping querier are enabled on the
switch. You assign a routing interface to the VLAN, with an IP address that
belongs to the same subnet as the multicast source and the host nodes.