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Administrator’s Guide for SIP-T4X IP Phones164ProcedureServer redundancy can be configured using the configuration files or locally.Configuration File .cfgConfigure the serverredundancy on the IP phone.For more information, refer toServer Redundancy on page338.Local Web User InterfaceConfigure the serverredundancy on the IP phone.Navigate to:http:///servlet?p=account-register&q=load&acc=0If a domain name is configured for a SIP server, the IP address(es) associated with thatdomain name will be discovered through DNS as specified by RFC 3263. The DNS queryinvolves NAPTR, SRV and A queries, which allows the IP phone to adapt to variousdeployment environments. The IP phone performs the NAPTR query for the SRV pointerand service type (UDP, TCP and TLS), the SRV query on the record returned from theNAPTR for the host name and the port number, and the A query for the IP addresses.If a port is set to 0 and the transport type is set to DNS-NAPTR, NAPTR and SRV querieswill be tried before falling back to A query. If no port is found through the DNS query,5060 will be used. If an explicit port (except 0) is specified and the transport type is setto DNS-NAPTR, the only lookup will be an A query.The following details the procedures of DNS query for the IP phone to resolve thedomain name of working server into the IP address, port and transport protocol.NAPTR (Naming Authority Pointer)First, the IP phone sends the NAPTR query to get the SRV pointer and service type. The IPphone performs a NAPTR query for the domain name. Example of NAPTR records:order pref flags service regexp replacementIN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.example.comIN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.com
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