IGMPv2 Fast Leave Support

IGMPv2 Fast Leave is a multicast mechanism for controlling how transmission is ceased for group multicast streams.  

Typically, when an IGMP snooper switch/router receives an IGMP Leave message, the snooper waits to receive a Membership Query message from the IGMP Querier to determine if there are any additional group members connected to the interface prior to removing the entry from the bridge table. 

When IGMP Fast Leave is enabled on a network multicast switch/router, the switch/router will automatically remove an interface from its bridge table upon receiving an IGMP Leave message regardless if there are additional clients in the multicast group which require service on the interface.  

In ePMP deployments, careful network multicast switch/router configuration must be observed, as network switches/routers with IGMP Fast Leave enabled can disrupt multicast application support for customers downstream.  

For example, consider the following scenario:

In this case, SM2 and SM3 belong to the same multicast group receiving IPTV multicast streams from the network Application Server.  When the Multicast Router situated above the AP is operating with IGMP Fast Leave enabled, if an IGMP Leave message is received from either of the streams served by SM2 and SM3, then the multicast stream's entry is automatically dropped from the switch/router port even though there are still members in the group.  This automatic bridge table removal at the Multicast Router can disrupt service to other customers in the sector (as the multicast group is no longer supported at the Multicast Router).

To help ePMP network operators to avoid this problem, the ePMP AP features a configuration parameter IGMPv2 Fast Leave Support.  This parameter, when enabled at the AP, will result in the AP dropping IGMPv2 Leave packets originating from registered subscriber modules if there are still other SMs subscribed to the multicast group downstream.  By dropping IGMPv2 Leave packets at the AP when there are other SMs in the downstream multicast group, the IGMPv2 Leave packets are not forwared upstream to the Fast Leave-enabled Multicast Router.  Since the Multicast Router does not receive the IGMPv2 Leave message, the multicast group is not blindly (regardless of downstream group members) terminated, and disruptions of customer application service are avoided.