Recently in troubleshooting an issue I saw some odd IGMP messages that I had never seen before. One of the record types was called “block-old-sources”. From RFC 3376 It states
BLOCK OLD SOURCES: Indicates that the Source Address [i] fields in this Group Record contain a list of the sources that the system no longer wishes to hear from, for packets sent to the specified multicast address. If the change was to an INCLUDE source list, these are the addresses that were deleted from the list; if the change was to an EXCLUDE source list, these are the addresses that were added to the list.
Unfortunately there were other devices that ALSO still wanted this multicast which was problematic. The device in question just kept sending this out for no real reason that we could discern.
It would be followed up by the message “all-new-sources” and “change-to-include-mode”
ALLOW NEW SOURCES : Indicates that the Source Address [i] fields in this Group Record contain a list of the additional sources that the system wishes to hear from, for packets sent to the specified multicast address. If the change was to an INCLUDE source list, these are the addresses that were added to the list; if the change was to an EXCLUDE source list, these are the addresses that were deleted from the list.
INCLUDE Mode: In INCLUDE mode, reception of packets sent to the specified multicast address is requested *only* from those IP source addresses listed in the source-list parameter. In EXCLUDE mode, reception of packets sent to the given multicast address is requested from all IP source addresses *except* those listed in the source-list parameter.
So basically it kept kind of killing the multicast and re-adding it, but it never really changed everything.