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Page 6 | AlliedWare™ OS How To Note: IGMP
IGMP overview > Messages
The key differences between a network’s Querier and its Snoopers are:
z The Querier generates Query messages to find out which ports need to transmit each
multicast stream. The Snoopers also use Query messages to find this out, but they use the
Querier’s messages—Snoopers cannot create Query messages themselves.
z The Querier has IGMP enabled as part of its IP configuration. Snoopers do not require any
configuration because snooping is enabled by default on Allied Telesis routers and managed
layer 3 switches.
z Querying is a layer 3 feature—the Querier looks into the IP headers of packets to
determine whether to forward them. IGMP snooping is a layer 2 feature. It does not
require an IP configuration.
Messages
The following table describes the different IGMP messages in more detail.
IGMP message types:
Membership Report
A client sends this when it wants to receive a multicast group. The Membership Report is
essentially a message that declares an interest in listening to a specified group.
Leave
A client sends this when it wants to leave a group.
General Query
The Querier sends this to all clients—whether or not the Querier is currently sending multicasts
to the client—to find out which groups they are listening to. Responses to General Queries ensure
that the Querier’s group membership information stays up to date.
The group address field for General Queries is set to 0.0.0.0. They are sent to a destination
address of 224.0.0.1, and by default Allied Telesis routers and switches send them every 125
seconds.
Specific Query
The Querier sends this to a group address, to check whether clients are still listening to that group.
The Querier sends a Specific Query after a client sends a Leave message for that group. Specific
Queries enable the Querier to confirm when all downstream clients have left a group, so that the
Querier can stop sending the multicast stream.
Membership Query
This is a general term for both Specific and General Queries.
Query Solicit
Switches send this when STP or EPSR detects a topology change. The Querier responds by sending
a General Query immediately instead of waiting until groups time out. This remaps IGMP to the
new topology as quickly as possible.