Allied Telesis AR400 manual Messages

IGMP overview > Messages

The key differences between a network’s Querier and its Snoopers are:

zThe 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.

zThe 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.

zQuerying 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.

Page 6 AlliedWare™ OS How To Note: IGMP