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Release 1.5 of the 4600 Series IP Telephones provided the capability to specify IP Address lists in either dotted decimal or DNS format. Release 1.5 allowed key network elements to have multiple IP Addresses, rather than being restricted to just one address for each element. You can specify up to 127 total characters in each list of the following devices:
Upon startup or a reboot, the 4600 Telephone attempts to establish communication with these various network elements in turn. The telephone starts with the first address on the respective list. If the communication is denied or times out, the telephone proceeds to the next address on the appropriate list and tries that one. The telephone does not report failure unless all the addresses on a given list fail.
Obviously, this capability significantly improves the reliability of IP telephony. Multiple IP Addresses maximize the telephone’s likelihood to communicate with backup equipment if the primary equipment is not operating or is not accessible. For example, alternate communication would be needed during a limited network outage.
However, this capability also has the advantage of making station number portability easier. Assume a situation where the company has multiple locations in London and New York, all sharing a corporate IP network. Users want to take their telephones from their offices in London and bring them to New York. When users start up their telephones in the new location, the local DHCP server will generally route them to the local call server. In this case the call server for H.323 is the Avaya Media Server and the call server for SIP is the Registration Server. But the local call server denies service because it knows nothing about these new users. With proper administration of the local DHCP server, the telephone knows to try a second call server IP Address, this one in London. The user can then be automatically registered with the London call server.
Chapter 4: Server Administration contains details on administration of DHCP servers for lists of alternate media servers, router/gateways, and TFTP servers. For specific information, see DNS Addressing.
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