Global SIP Settings41-001160-00, Release 2.1, Rev 04 4-49Configuring Network and Session Initiation Protocol (SIP) FeaturesConfiguring the IP PhonesBackup Proxy/Registrar SupportThe IP phones support a backup SIP proxy and backup SIP registrar feature. Ifthe primary server is unavailable, the phone automatically switches to the backupserver allowing the user's phone to remain in service.How it WorksAll SIP registration messages are sent to the primary registrar first. If the server isunavailable, then a new registration request is sent to the backup registrar. Thisalso applies to registration renewal messages, which try the primary server beforethe backup.Similarly, any outgoing calls attempt to use the primary proxy first, then thebackup if necessary. In addition, subscriptions for BLF, BLA, and explicit MWIcan also use the backup proxy when the primary fails. Outgoing calls and thepreviously mentioned subscriptions behave the same as registrations, where theprimary proxy is tried before the backup.You can configure the backup SIP proxy on a global or per-line basis via theconfiguration files or the Aastra Web UI.SIP Server (SRV) LookupThe SIP SRV Lookup feature allows you to configure the IP phone to perform aDNS 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 FQDNand 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 multiple servers, based onthe 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 isnon-zero, then the phone performs a DNS "A" Name Query to resolve the FQDNinto dot notation form.If the IP address is a valid dot notation and the port is zero, then a default port5060 is used.
