How to implement DHCP server with fail-over high availablity

[Joe Shen]

hi，

  we want to set up DHCP service with 99.999% availiablity.

  In order to reach this target, we want to set up dhcp cluster 

  on different physical sites. Even when one site is totally down,

  our customer's connection should be kept, and new connection request 

  should be serviced by other sites.  

  On the other hand,  load should be balanced between different clusters.

  Could  anybody do me a favor on giving some advice?

each word will be highly appreciated.

Joe