If a node is added as "External" we do not update any information regarding it. If we wish to update IP dynamically, we should set as ICMP.
“The External status is reserved for nodes hosting applications that are to be monitored with Orion Application Performance Monitor. Orion will not collect or monitor any data about a node itself, if it is marked as External.” (http://www.solarwinds.com/documentation/en/flarehelp/orionplatform/content/orioncoreagaddingdeviceswebconsole.htm))
It's not intuitive and we certainly could adjust this behavior. Will look to do so in future releases.