Aastra 9000i Series Administrator's Manual page 87

Configuration Server Redundancy via DNS A Records
The phone sends a DNS query and in the DNS response, it accepts the first server IP address and
contacts that server, ignoring any additional IP addresses in the response. This allows service
providers to manage load balancing (via the DNS server putting different records first on each
request), but does not provide redundancy.
The phones also provide support of multiple IP addresses being returned for the DNS lookup for
server redundancy via multiple DNS A record entries. The phone tries to contact the first server
address it receives, but if this fails, it now tries to contact the second server address, etc.
This feature supports all the download protocols (TFTP, FTP, HTTP, and HTTPS).
Limitation
• In certain cases, the TFTP Protocol cannot distinguish between "server down" and "no file
on server" error messages; therefore, the failover in these instances may fail.
2-20
Notes:
1.
Once the phone has failed over to a redundant server, it
continues to use that server for all other server-related processes on the
phone (i.e., firmware upgrades from the Web UI, boot-up process,
etc.).
2.
If a server fails while downloading a file(s) to the phone, the
phone performs the discovery process of finding a redundant server
that is available. When the boot is complete on the redundant server,
the phone tries to download the file(s) again from the previous server.
The check-sync process also performs the same way when a server
fails.
3.
The "Skip" softkey displays in the event of a network outage,
the user can skip the configuration download and continue the boot.
4.
All server failovers and failed server IP addresses are logged in
the "Error Messages" page on the IP Phone UI at Options->Phone
Status->Error Messages.
41-001343-01 Rev 02, Release 3.2.2