Configuring Advanced Features
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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:
1. Sends the INVITE request to the primary server.
2. If the primary server does not respond correctly to the INVITE (that is, the primary server
responds to the INVITE with 503 message or the request for responding with 100 Trying
message times out (64*T1 seconds, defined in RFC 3261)), then tries to make the call using
the secondary server.
3. If the secondary server is also unavailable, the IP phone will try the fallback server until it
either succeeds in making a call or exhausts all servers at which point the call will fail.
At the start of a call, server availability is determined by SIP signaling failure. SIP signaling failure
depends on the SIP protocol being used as described below:
If TCP is used, then the signaling fails if the connection or the send fails.
If UDP is used, then the signaling fails if ICMP is detected or if the signal times out. If the
signaling has been attempted through all servers in the list (this list contains all the server
addresses resolved by the DNS server) and this is the last server, then the signaling fails
after the complete UDP timeout defined in RFC 3261. If it is not the last server in the list,
the maximum number of retries depends on the configured retry counts (configured by
the parameter “account.X.sip_server.Y.retry_counts”).
Phone Registration
Registration method of the failover mode:
The IP phone must always register to the primary server first except in failover conditions. If this
is unsuccessful, the phone will re-register as many times as configured until the registration is
successful. When the primary server registration is unavailable, the secondary server will serve as
the working server. As soon as the primary server registration succeeds, it returns to being the
working server.
Registration methods of the fallback mode include (not applicable to outbound proxy servers):
Concurrent registration (default): The IP phone registers to SIP server 1 and SIP server 2
(working server and fallback server) at the same time. Note that although the IP phone
registers to two SIP servers, only one server works at the same time. In a failure situation, a
fallback server can take over the basic calling capability, but without some advanced
features (for example, shared lines and MWI) offered by the working server.
Successive registration: The IP phone only registers to one server at a time. The IP phone
first registers to the working server. In a failure situation, the IP phone registers to the
fallback server, and the fallback server can take over all calling capabilities.
For more information on server redundancy, refer to
Server Redundancy on Yealink IP Phones.
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