Question:
When does NetScaler send a HTTP error : 503 Service Unavailable when accessing an FQDN/URL through Content Switch?

Answer:
NetScaler will send an HTTP 503 error in the below conditions when accessing the Content Switch:
1. The Load Balancing Vserver for which policy is matched is Down or Out of Service.
2. No Content Switch policy is matched and no Default LB Vserver is configured.