Allied Telesis AR400 manual Explanation of IGMP snooping, process

IGMP snooping > Explanation of IGMP snooping

Explanation of IGMP snooping

This section steps through the events that occur in a typical use of multicasting in this network: to stream multicast packets for a group.

IGMP The following figure shows the process by which IGMP tracks multicast clients and ensures learning that the correct clients receive the stream.

process

What happens before a multicast client exists:

1.Querier starts receiving multicast stream from server. Querier has no interested clients so doesn’t forward multicast.

Multicast	49	26
		3

Querier	Blocked	Snooper 2	Snooper 1
	by STP
			Pontential
Multicast
			client
server

GQ

2.Querier sends General Query (GQ)

to find out if any would-be clients exist. Snooper 1 receives Query on port 26, snoops Query, and creates All Groups entry for port 26.

3.Snooper 1 forwards Query out all ports. No clients exist, so no clients reply to Query.

Multicast 49

Querier

Multicast 49

Querier

26

3 Snooper 1

GQ 26 GQ

3 Snooper 1

What happens when a client joins a multicast group:

4.Client joins group by sending Membership Report to the group address.

5.Snooper 1 receives Report on port 3, snoops Report, and adds a group entry for port 3. Snooper 1 forwards Report out its All Groups port.

6.Querier receives Report on port 49 and adds a group entry for port 49. Querier starts forwarding multicast stream out port 49.

7.Snooper 1 receives multicast stream and forwards it out port 3.

8.Querier continues to send General Queries periodically. These keep All Groups entries alive on Snoopers.

Multicast	49	26	Report
			3
	Querier	Snooper 1
			Client
		Report
Multicast	49	26
			3
	Querier	Snooper 1
		Multicast
Multicast	49	26
			3
	Querier	Snooper 1
		Multicast
Multicast	49	26	Multicast
			3
	Querier	Snooper 1

	GQ	Multicast
Multicast	49	26	Multicast
			3
	Querier	Snooper 1

igmp-join.eps

Page 11 AlliedWare™ OS How To Note: IGMP