Cut-off or troubles may occur on IP multicast while unicasting still operational. This greatly affects system stability.
To reproduce it: On Cisco switch, commands like "no ip multicast-routing" or "no ip multicast-routing distributed" disable multi-cast support.
Analysis: When 'B' misses heartbeat (multi-cast message) coming from 'A', it marks it as suspected (failure detection protocol) then notify VERIFY_SUSPECT. This protocol checks if 'A' is really DEAD. This is done by sending (unicast) ARE_YOU_DEAD request to suspected member. 'A' sends back response I_AM_NOT_DEAD (unicast again). Thus, members 'A' and 'B' remain in the same group in spite of being "multicastly" disconnected.
Expectation: We expect that a member that belongs to a view can send unicast and multicast message to other members of the same view. So if one support is broken, unreachable members should be excluded.
Improvement : VERIFY-SUSPECT protocol checks both unicast and muticast support: suspected member sends back I_AM_NOT_DEAD message using IP multicasting