User guide

Old IGMP Snooping Functionality IGMP snooping
3-15
iMG/RG Software Reference Manual (IGMP)
Upon receiving an IGMP Leave message, the system can either process it immediately - as described above (This
is known as FastLeave) - or if configured to do so - send an IGMP Group Specific Query to the port where the
IGMP Leave message was received from. The Leave Time value is used in the query message to request a fast
response from other hosts that may be present on the same Ethernet segment. This can be used to ensure that
when one host asks for a multicast stream to be stopped - it does not adversely impact another host on the
same port that is subscribed to that multicast stream.
If no answer is received to the IGMP Group Specific Query and if no other ports have hosts joined to the same
multicast group, then an IGMP leave messages is sent to the multicast router. In this way the multicast traffic the
router is asked to stop sending any multicast data for that particular group.
The IGMP leave message forwarded by the Gateway will have as source MAC address the Gateway's MAC
address and will have as source IP address the ipaddress of the ip interface associated with the VLAN that is
associated with the IGMP service.
3.1.3.4 IP source address masking – Secondary IP Interface
If the Interface associated with the VLAN that the IGMP module is associated with does not have an IP address,
it is possible to refer, as source IP address for upstream IGMP signalling messages, the IP address of any other
existing IP interface. This interface is not required to be attached to the VLAN where IGMP snooping has been
enabled.
3.1.3.5 IGMP snooping security
This feature allows the iBG/iMG/RG to limit accepted IGMP signalling to that from designated STB identified by
their MAC addresses. These MAC addresses will be learned automatically by the software up to a configured
number and saved in a non volatile memory. They are specifically named in the configuration. with the maximum
number of STB MAC addresses supported being 10. It is possible to manually configure the allowed MAC
addresses - so that via a provisioning action - the security of the Video network is maintained.
3.1.3.6 Routed IGMP proxy
An alternative to Bridged IGMP snooping is routed IGMP.
This is a layer-3 feature that allows multicast traffic to be routed between multiple IP interfaces.
IGMP traffic is typically limited to the VLAN where it is received. If a host joins a multicast group but multicast
traffic is received on another VLAN to which the host is not connected, the multicast traffic will never reach the
host.
Routed IGMP overrides this limitation with the only constraint that multicast traffic must be received only on
one IP interface called the upstream interface.
In this case, when a host joins a multicast group, the IP interface attached to the transport (VLAN) where the
host is located, becomes a downstream interface. It will receive all the multicast traffic related to the group that
the host has joined.