Figure 5 illustrates IGMP snooping, in which an intermediate device (such as a DSLAM) transparently monitors IGMP traffic. The device adds interfaces to its outgoing interface table when it detects join request messages and removes interfaces from its outgoing interface table when it detects leave request messages. The snooping device also maintains state information for general membership query maximum response time timers if the IGMP client does not issue a leave message (for example, if an IPTV set-top box experiences a power outage).
Figure 5: IGMP Snooping
Because IGMP snooping is transparent, the snooping device typically does not participate in IGMP host messaging. The device only monitors transactions between clients and routers, forwarding IGMP packets upstream to the multicast router and determining when join or leave processing is required for a downstream host. One exception to this transparency occurs when the snooping device intercepts membership reports based on local filters to prevent the host from joining specific groups (that is, specific broadcast channels allocated to multicast groups that are blocked from being received by the set-top box).
The snooping device can receive multicast data in several ways within a broadband access network. The router might be configured to flood all multicast groups downstream to the snooping device. The upstream router might forward only groups based on IGMP membership reports that it receives from the IGMP hosts. The snooping agent might invoke an IGMP client process to source its own membership reports that it sends to the multicast router, and so on. However, these various options are beyond the scope of this document.
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