Aastra Telecom C53I User Guide

Advanced Network Settings

4-46

41-001160-00, Rev 01  Release 2.0  

IP Phone Administrator Guide

The IP phones support a backup SIP proxy and backup SIP registrar feature.  If 
the primary server is unavailable, the phone automatically swtiches to the backup 
server allowing the user's phone to remain in service.

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 registraion 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.

The SIP SRV Lookup feature allows you to configure the IP phone to perform a 
DNS server lookup on a SIP proxy, a SIP registrar, or a SIP outbound proxy.

The IP phone performs an SRV lookup when the IP address of the server is FQDN 
and the corresponding port is 0.

For example, if the phone is configured with sip proxy ip of "ana.aastra.com", 
and sip proxy port of "0", the SRV lookup may return multilple servers, based on 
the priorities if one is selected as primary and others are selected as secondary.

However, if the IP address is an FQDN and the corresponding server port is 
non-zero, then the phone performs a DNS "A" Name Query to resolve the FQDN 
into dot notation form.

If the IP address is a valid dot notation and the port is zero, then a default port 
5060 is used.