Database Mirroring: naming resolution issues during failover connectivity

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 DOS prompt, type "nslookup (IP address)" and hit enter. You should first get IP and name of your own DNS server, then the IP and name of the IP you looked up. If you were to do this at FCNY, for instance. Refer to this SQL Server Connectivity article on troubleshooting this error.

Whereas in a database Mirroring session the LMHOSTS file cannot be used for the name resolution, in this case if DNS is an issue then use IP address instead. Name Resolution is only required for adding a Witness server to the Database mirroring session. When you use database mirroring in high-availability mode, the process of determining a failover is based on the network connection. If there is a problem with the network, mirroring will fail over or deny access to the database because of the quorum requirement.

Few FAQs on Database Mirroring: In the case of server having multiple network adapters in the server and if you need to dedicate to mirroring, associate a specific IP address to that adapter.  There are no specific restrictions on the network for mirroring, but the network connection between the servers is critical. The network should generally be dedicated, and be of high quality and high bandwidth. As a rough guideline, the network bandwidth should be three times the maximum log generation rate.