Why Hypervisor got fenced in Red Hat Enterprise Virtualization?

Issue

• Why Hypervisor got fenced in Red Hat Enterprise Virtualization?
• /var/log/ovirt-engine/engine.log:
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2013-09-18 15:48:52,562 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.VdsBrokerCommand] (QuartzScheduler_Worker-63) XML RPC error in command ListVDS ( Vds: xxxx.acxxxx.local ), the error was: java.util.concurrent.ExecutionException: java.lang.reflect.InvocationTargetException 
2013-09-18 15:48:52,572 WARN  [org.ovirt.engine.core.vdsbroker.VdsManager] (QuartzScheduler_Worker-63) ResourceManager::refreshVdsRunTimeInfo::Failed to refresh VDS , vds = 2xxxx346-4af8-11e1-8bc4-00224d673da0 : ackxxx.acxxxy.local, VDS Network Error, continuing.
VDSNetworkException: 
[...]
<time> INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.FenceVdsVDSCommand] (pool-3-thread-5) [3ec55051] START, FenceVdsVDSCommand(HostName = xxxxx, HostId = xxxxxxx, targetVdsId = yyyyyy, action = Stop, ip = xx.xx.xx.xx, port = , type = ipmilan, user = ADMIN, password = ******, options = ''), log id: 303249ca  <===