Yealink SIP-T54S Administrator's Manual page 776

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Phone Configuration for Redundancy Implementation
To assist in explaining the redundancy behavior, an illustrative example of how an IP phone
may be configured is shown as below. In the example, server redundancy for fallback and
failover purposes is deployed. Two separate servers (a working server and a fallback server) are
configured for per line registration.
Working Server: Server 1 is configured with the domain name of the working server. For
example: yealink.pbx.com. DNS mechanism is used such that the working server is resolved to
multiple servers with different IP addresses for failover purpose. The working server is deployed
in redundant pairs, designated as primary and secondary servers. The primary server (e.g.,
192.168.1.13) has the highest priority server in a cluster of servers resolved by the DNS server.
The secondary server (e.g., 192.168.1.14) backs up a primary server when the primary server
fails and offers the same functionality as the primary server.
Fallback Server: Server 2 is configured with the IP address of the fallback server. For example,
192.168.1.15. A fallback server offers less functionality than the working server.
Outgoing Call When the Working Server Connection Fails
When a user initiates a call, the IP phone will go through the following steps to connect the call:
Sends the INVITE request to the primary server.
1.
If the primary server does not respond correctly to the INVITE (that is, the primary server
2.
responds to the INVITE with 503 message or the request for responding with 100 Trying
message times out (64*T1 seconds, defined in
the secondary server.
If the secondary server is also unavailable, the IP phone will try the fallback server until it
3.
either succeeds in making a call or exhausts all servers at which point the call will fail.
750
RFC 3261)), then tries to make the call using