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Corosync and ifdown on active network interface
How does ifdown of an active interface work?
The short answer is poorly.
Corosync monitors interfaces it is using looking for an event indicating the interface has been taken out of service. When it does this, it rebinds corosync to the localhost IP address. The reason it does this is some individuals want to use Corosync locally with only 1 node, and then when a network interface becomes available, form a proper cluster.
Other people want to test how split brain works and ifdown the local ip. This causes third party applications to behave erratically although corosync alone generally tolerates this well.
If you want to simulate split brain, use iptables to block the incoming and outgoing udp traffic on the port.
Finally we do have plans to address this problem so that the users of Corosync can be satisified in the two use cases defined above.