Membership arbitration is necessary on a perceived membership change because systems may falsely appear to be down. When LLT on a system no longer receives heartbeat messages from another system on any configured LLT interface, GAB marks the system as DOWN. However, if the cluster interconnect network failed, a system can appear to be failed when it actually is not. In most environments when this happens, it is caused by an insufficient cluster interconnect network infrastructure, usually one that routes all communication links through a single point of failure.
If all the cluster interconnect links fail, it is possible for one cluster to separate into two subclusters, each of which does not know about the other subcluster. The two subclusters could each carry out recovery actions for the departed systems. This is termed split brain.
In a split brain condition, two systems could try to import the same storage and cause data corruption, have an IP address up in two places, or mistakenly run an application in two places at once.
Membership arbitration guarantees against such split brain conditions.