Aastra 9143i Series Administrator's Manual page 258

IP Phone Administrator Guide
Global SIP Settings
Backup Proxy/Registrar Support
The IP phones support a backup SIP proxy and backup SIP registrar feature. If
the primary server is unavailable, the phone automatically switches to the backup
server allowing the user's phone to remain in service.
How it Works
All SIP registration messages are sent to the primary registrar first. If the server is
unavailable, then a new registration request is sent to the backup registrar. This
also applies to registration renewal messages, which try the primary server before
the backup.
Similarly, any outgoing calls attempt to use the primary proxy first, then the
backup if necessary. In addition, subscriptions for BLF, BLA, and explicit MWI
can also use the backup proxy when the primary fails. Outgoing calls and the
previously mentioned subscriptions behave the same as registrations, where the
primary proxy is tried before the backup.
You can configure the backup SIP proxy on a global or per-line basis via the
configuration files or the Aastra Web UI.
SIP Outbound Support
The IP Phones support draft-ietf-sip-outbound-15. That specification describes
how a SIP User Agent (UA) behind a firewall, reuses an existing connection
(usually the REGISTER outbound connection) for the inbound request if the
proxy supports it. The UA uses keep-alive packets to monitor the connection
status.
An Administrator can enable or disable this feature using the following parameter
in the configuration files:
• sip outbound support
4-74
Note: TLS and TCP both support this feature. If the Global SIP
parameter "Persistent TLS" is set on the phone, then only one TLS
persistent connection can be established since the phone uses the local
port 5061 for connection. If the Global SIP parameter "TLS" is set on
the phone, more than one connection can be setup since the phone uses
a random local port for connection.
41-001160-03 Rev 00, Release 2.5