DHCP server fail-over and accounting

[Joe <sj_hznm () hotmail com>]

hi,
 
    we plan to implement DHCP server farm in our network.   Currently , there are there  problems burning my head. 
could anybody 
    do some help?
 
 
   1.  How to set up DHCP server farm with high availability? 
 
       It's required to set up DHCP server with 99.999% available. 
 
       To our experience, this needs to set up  DHCP  server on two sites and syncronize their content in real time.
 
      Beside this ,  we hope  there should be as less modification as possible  on edge router when one DHCP  server is 
down.
 
      should anycast architecture helpful ?   or should we just set up two dhcp servers on two sites and  sync. with 
ISC DHCPD?
 
      is there any other  method for fail-over and high availability?
 
 
  2. How to set up accouting and authentication with DHCP?
 
     Regulation and content based pricing is demanded.  So, we plan to authenticate customer and accouting on their 
usage.
 
     In previous list post,  someone said  Juniper could do radius before DHCP.  Does this mean  Juniper could 
authenticate 
 
     user with radius (username/password) before DHCP ?  Is there anyway to collect user traffic under DHCP ? 
 
   
3.  Someone said PPPOE is not good for customer looking for long time online ,  DHCP is an good option.  But, to my 
understanding 
 
     DHCP is just good for those looking for easy-transfer to IPv6  , because pppoe could also make user on line as 
long as possible.
 
    Is there any reference on DHCP server consideration on 4-to-6 migration?
 
 
 Joe