User guide

IGMP snooping IGMP snooping Functional Overview (Includes
iMG/RG Software Reference Manual (IGMP)
3-6
3.1.2.2.2 Leaving a multicast group
Periodically the multicast router sends Generic Queries to check whether there are multicast hosts that are
still active.
If one or most hosts are still interested to receive multicast streams, they will reply with IGMP Report mes-
sages and the corresponding entries on the local multicast group database will be refreshed.
When an host wants to leave group, it sends an IGMP Leave message specific for the group it wants to leave.
The IGMP Leave message is then forwarded to the upstream multicast router and a timer equals to the Last
Member Query Interval secs is started for the corresponding local igmp entry.
When this timer expires, the IGMP process stops the forwarding of the multicast stream on the port that has
received the IGMP leave message.
This mechanism is used to reduce the flooding of unsolicited multicast streams in case the multicast upstream
router takes a long time before closing the multicast stream towards the Residential Gateway.
The upper multicast router can decide to keep open the multicast stream towards the Residential Gateway if it
has detected that there are other hosts interested to receive the multicast stream.
This is usually done by the upper multicast router upon the reception of an IGMP leave messages sending one
or more specific queries for the multicast stream just left.
3.1.2.3 Proxy Operational Mode
Proxy Mode is an operational mode where the Residential Gateway takes a more active roll in the management
of the IGMP messages.
IGMP messages received from the upper multicast router or from the internal hosts are always terminated into
the Residential Gateway.
IGMP messages sent by the Residential Gateway to the internal hosts or to the upper multicast router will use
the CPE source IP and MAC addresses creating in this way a demarcation point between the access and the
user network.
3.1.2.3.1 Joining a Multicast Group
As for IGMP Snoop-Only mode, the system listens for unsolicited IGMP Report messages that hosts send to
join a multicast group.
The Residential Gateway updates the local multicast group database storing the information about the
requested stream and the requesting port.
If the received IGMP report message is the first one (i.e. no other hosts have requested the same multicast
stream), then the IGMP process forwards immediately the IGMP Report message to the upper multicast router
(replacing the source IP and MAC addresses).