GeoDNS (or GeoIP) is a patch for BIND DNS server software, to allow geographical split horizon (different DNS answers based on client's geographical location), based on MaxMind's geoip (commercial) or geolite (free) databases.
The objective of this technology is to enhance the DNS resolution based on the geographical location of the client.
It does not require any support from the ISP and will not break existing connections when the server selected for a particular client changes.
However, as it is not intimately tied into the network infrastructure it is likely to be less accurate at sending data to the nearest server.
As ISPs typically arrange for users to use DNS servers geographically near them, the system usually works nonetheless.