19216811.liveIGMP snooping
IGMP snooping Functional Overview (Includes
iMG/RG Software Reference Manual (IGMP)
3-4
The following sections describe the IGMP snooping functionality for iMG models belonging to group Fiber-B,
Fiber-D, Fiber-E, Modular and ADSL-B, ADSL-C.
3.1.2.1 Multicast router port discovery
The system listens for IGMP General Query messages and records the port(s) where any such message has
been received.
In this way the Gateway knows where multicast routers are located in order to forward IGMP report and leave
messages only to the correct uplink port(s).
Once the Residential Gateway has detected where the multicast router is located, it keeps the entry for a
period of time defined by the Bridge Multicast Interface Aging Time attribute.
If a new IGMP General Query is received, the multicast router timer is refreshed and the corresponding uplink
port is updated if needed.
If the multicast entry expires before any IGMP General Query is received, forwarding of any multicast stream
to internal hosts is stopped.
It's therefore recommended that the multicast uplink interface timer is longer than the query interval config-
ured on the multicast router (two times the query interval, at least).
Then the forwarding of IGMP queries from multicast router and the forwarding of IGMP report/leave messages
from internal multicast hosts follows different schemes depending if the IGMP process on the
Residential Gateway is working in Snoop-Only mode or it is configured to work in Proxy mode.
Independently on the operational mode, the IGMP process on the Residential Gateway keeps always a view of
the multicast network updating the local multicast group database
3.1.2.2 Snoop-Only Operation Mode
Snoop-Only mode is the default operational mode for IGMP snooping. It's possible to force the IGMP snooping
to work in Snoop-Only mode via the bridge set igmp snooping mode snooponly command.
Before changing the igmp operational mode it's always recommended to disable the IGMP process via the
bridge set igmpsnooping disable command and then re-enable it after the configuration changes have been
entered.
When operating in Snoop-Only mode, the IGMP process does not act any change on IGMP messages. IGMP
source IP and MAC addresses are left unchanged and they are forwarded through the Residential Gateway as
they arrive to the CPE.
IGMP process checks only if there are hosts that have joined or left multicast streams in order to update the
local multicast group database.
The following picture shows an example of IGMP messages flow when Snoop-Only mode is active.