OnCommand does auto-discovery which by default is turned on. Do check whats your values for this options.
Also when adding a controller, the corresponding subnet is also added, unless you use the -N option in the add command.
[root@10 ~]# dfm options list | grep -i discover
discoverAgents Enabled
discoverClusters Enabled
discoverEnabled Enabled
discoverHostInitEnabled Enabled
discoverHosts Enabled
discoverInterval 15 minutes
discoverNetworks Disabled
discoverSAN Enabled
discoverTimeout 5 seconds
discoverVfilers Enabled
Regards
adai