DNS iRule questions

GTM is a fancy DNS box. After all the intelligence, only one thing happens, an IP address (or more, depending on configuration) is returned to the device that requested one. So if DataCenter1_vip is down, and the local dns server of the client has that address cached for 10 minutes, then all clients utilizing that ldns will be down for the remaining TTL. That downtime can be mitigated on the LTM hosting that *down* service if the path to it from the client is still up (ie, only the application itself has failed, the infrastructure is still intact). On the LTM, you could redirect traffic over to the other site, either by configuring the http profile with a fallback host, or if the logic is more complicated, utilize an iRule.