DHCP Redundancy

the Radio theradio at free.fr
Sun Nov 28 10:49:10 UTC 2010

"If one fails, the other can take over and manage the full address space."

Be warned that only HALF of the address space is "automatically" taken 
over by the surviving DHCP process (assuming the failover team consists 
of 2 servers) :
/"If one server fails, the other server will continue to renew leases 
out of the pool, and will allocate new addresses out of the roughly half 
of available addresses that it had when communications with the other 
server were lost. "/

v4.2.0-P1 option "auto-partner-down" does not work for now, then 
imposing to MANUALLY edit the leases file to set the surviving server 
into "PARTNER-DOWN" state (see dhcpd.conf help file).

Therefore, the failover efficiency depends greatly on the clients 
requests rate : I had a big wireless infrastructure that, whenever a 
controller shuts down, half of the 1000 APs request their address back - 
until you have a more than 2000 addresses subnet, some APs do not get 
their IP.

My main advice concerning failover (until partner-down state is really 
automatized) : use subnets large as twice as needed (if you have 200 
expected clients, set for a /23 subnet at least).

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