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Page 271 / 998 Scroll up to view Page 266 - 270
IGMP snooping Functional Overview (Includes New Functionality)
IGMP snooping
3-9
iMG/RG Software Reference Manual (IGMP)
3.1.2.3.2 Leaving a Multicast Stream
Under Proxy operational mode, when an host wants to leave a multicast group and sends an IGMP Leave mes-
sage, the IGMP process takes different actions depending if the Fast Leave feature is enabled or disabled.
•
Fast Leave Disabled
Upon the reception of an IGMP leave message, the IGMP process starts sending IGMP Specific Queries to the
port that has received the leave message to double check whether that there are other hosts still interested to
receive the multicast stream.
The number of IGMP specific queries sent by the Residential Gateway is defined by the Robustness attribute.
The max response time that the IGMP process wait for an answer is defined by the Last Member Query Interval
value.
If no hosts answer to the Residential gateway in a timeframe less than Last Member Query Interval times the
Robustness variable, the Residential Gateway will purge from the local igmp database the entry that matches the
multicast stream and the corresponding port.
Then, if there are no other hosts on the other ports that are listening the same multicast stream, the IGMP pro-
cess will send an IGMP leave message to the multicast router to inform it that it can close the multicast stream
towards the Residential Gateway.
The picture here below shows an example scenario where two hosts join two different multicast channels.
Page 272 / 998
IGMP snooping
IGMP snooping Functional Overview (Includes
iMG/RG Software Reference Manual (IGMP)
3-10
FIGURE 3-3
Two Hosts Join Two Different Multicast Channels
In case a multicast host is disconnected from the network, the IGMP process is able to detect such condition
checking the absence of IGMP reports on the port where the host left.
This process takes a time that is usually longer than the case where the host leaves the network in a gracefully
way. The IGMP process has to wait for no answers to the internal Generic Queries a number of times equals to
the Robustness attribute value.
The picture here below shows an example where an host disconnects from the network without sending any
IGMP leave message.
Page 273 / 998
IGMP snooping Functional Overview (Includes New Functionality)
IGMP snooping
3-11
iMG/RG Software Reference Manual (IGMP)
FIGURE 3-4
Host Disconnects - No Leave Message
•
Fast Leave Enabled
When Fast Leave support is enabled, upon the reception of an IGMP leave message, the IGMP process stops
immediately the forwarding of multicast stream towards the internal host.
The IGMP process does not send any specific query to check if there are other hosts still interested to receive
the multicast stream.
When the IGMP process receives the IGMP leave message, if there are no other hosts receiving the same
stream on other ports, it sends immediately an IGMP leave message to the multicast router.
Page 274 / 998
IGMP snooping
IGMP snooping Functional Overview (Includes
iMG/RG Software Reference Manual (IGMP)
3-12
In case other hosts have joined the same multicast stream, the IGMP process purges only the entry matching
the corresponding lan port and drop the IGMP leave message.
The picture here below shows an example scenarios where an host leaves a multicast stream and a scenario
where two hosts leave the same multicast stream.
FIGURE 3-5
One and Two Hosts Leave the Same Multicast Stream
Page 275 / 998
Old IGMP Snooping Functionality
IGMP snooping
3-13
iMG/RG Software Reference Manual (IGMP)
3.1.3
Old IGMP Snooping Functionality
The following sections describe the IGMP snooping functionality for iMG models belonging to group Fiber-A,
Fiber-C, and ADSL-A.
3.1.3.1 Multicast router port discovery
IGMP snooping is activated using the IGMP SNOOPING ENABLE command.
The system listens for IGMP Membership General Query packets sent to the address 01-00-5e-00-00-01 and
records the port(s) where any such message has been received.
In this way the Residential Gateway knows where multicast routers are located in order to forward report and
leave messages only to the correct port(s).
Note that even if multiple VLANs can be present in the system, the IGMP snooping feature can be turned on
only on one VLAN at time.
3.1.3.2 Snoop-Only Operation Mode
3.1.3.2.1 Joining a Multicast Group
The system listens for unsolicited IGMP Report messages that hosts send to join a multicast group and records
the port where each message has been received. What happens next depends on the circumstances in which
the packet is received. To understand this, let us consider two possible scenarios:
•
First Scenario:
Host A is the first host in an Ethernet segment to join a group.
Host A sends an unsolicited IGMP Membership report.
The Residential Gateway intercepts the IGMP membership report sent Host A and creates a multicast entry for
the group that host A was requesting. It then links this entry to the port on which it has received the report.
It also sets, for this port and this multicast group , a local Timeout timer to the Timeout Interval value This
timer is used to refresh the multicast membership table periodically.
The system then forwards the IGMP report on to the multicast router. In this way the router will also receive
the IGMP report and will update its multicast routing table accordingly. If no Multicast router has been detected,
then it does nothing.
Immediately multicast traffic for the requested group address is forwarded only to the port where the report
from Host A has been received.
•
Second Scenario:
Another host B, on the same Ethernet segment as host A joins the same multicast group as host A.

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