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Page 11 | AlliedWare™ OS How To Note: IGMP
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
learning
process
The following figure shows the process by which IGMP tracks multicast clients and ensures
that the correct clients receive the stream.
1. Querier starts receiving multicast stream
from server. Querier has no interested
clients so doesn’t forward multicast.
Snooper 2
Snooper 1
Querier
Multicast
server
Pontential
client
Multicast
26
Blocked
by STP
49
3
What happens before a multicast client exists:
What happens when a client joins a multicast group:
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.
Snooper 1
Querier
GQ
26
49
3
Multicast
3. Snooper 1 forwards Query out all ports.
No clients exist, so no clients reply to
Query.
Snooper 1
Querier
GQ
26
49
3
GQ
Multicast
4. Client joins group by sending Membership
Report to the group address.
Client
Snooper 1
Querier
Report
26
49
3
Multicast
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.
Snooper 1
Querier
Report
26
49
3
Multicast
6. Querier receives Report on port 49
and adds a group entry for port 49.
Querier starts forwarding multicast
stream out port 49.
Snooper 1
Querier
Multicast
26
49
3
Multicast
7. Snooper 1 receives multicast stream
and forwards it out port 3.
Snooper 1
Querier
Multicast
26
49
3
Multicast
Multicast
8. Querier continues to send
General Queries periodically.
These keep All Groups entries
alive on Snoopers.
Snooper 1
Querier
Multicast
26
49
3
Multicast
GQ
Multicast
igmp-join.eps