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Release 1.5 of the 4600 Series Telephones provided the capability to specify IP address lists (in either dotted decimal or DNS format) for key elements of the network, rather than merely one address for each. Specifically, you can specify up to 127 total characters in each list of the following: router/gateways, TFTP servers, and the media server. When the 4600 telephone is powered up or is rebooted, it attempts to establish communication with these various network elements in turn, starting 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 have failed.
Obviously, this capability can significantly improve the reliability of IP telephony by maximizing the likelihood of the telephone communicating with backup equipment if the primary equipment is down or inaccessible (say, perhaps due to 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 (for example, 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 power up their telephones in the new location, the local DHCP server will generally route them to the local switch, which denies service because it knows nothing about these new users. However, with proper administration of the local DHCP server, the telephone knows to try a second media server IP address, this one in London. The user can then be automatically registered with the London switch.
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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