Within a Database Mirroring session within an automatic failover situation it may fail with the error : "The server network address "%.*ls" can not be reached or does not exist. Check the network address name and reissue the command". If you suspect DNS as a culprit for failure to connect, then perform a PING against that Server is a good start for troubleshooting. But there are some other quick tests you can try as well, such as "nslookup", which lets you resolve an address by name or IP. From a...(
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Read the complete post at http://sqlserver-qa.net/blogs/ha/archive/2007/09/24/database-mirroring-naming-resolution-issues-during-failover-connectivity.aspx