Yealink CP969, Руководство администратора

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Configuring Advanced Features
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Server Redundancy
Server redundancy is often required in VoIP deployments to ensure continuity of phone service,
for events where the server needs to be taken offline for maintenance, the server fails, or the
connection between the IP phone and the server fails.
Two types of redundancy are possible. In some cases, a combination of the two may be
deployed:

Failover:
In this mode, the full phone system functionality is preserved by having a second
equivalent capability call server take over from the one that has gone down/off-line. This
mode of operation should be done using the DNS mechanism from the primary to the
secondary server. Therefore, if you want to use this mode, the server must be configured
with a domain name.

Fallback:
In this mode, a second less featured call server with SIP capability takes over call
control to provide basic calling capability, but without some advanced features (for
example, shared line and MWI) offered by the working server. IP phones support
configuration of two servers per SIP registration for fallback purpose.
Note
For concurrent registration mode, it has certain limitation when using some advanced features,
and for successive registration mode, the phone service may have a brief interrupt while the
server fails. So we recommend you to use the failover mode for server redundancy because this
mode can ensure the continuity of the phone service and you can use all the call features while
the server fails.