Preserving lease database after enabling failover feature

Simon Hobson dhcp at
Tue May 30 07:10:50 UTC 2006

Usman Wahid wrote:

>I have a setup of two DHCP servers running the latest 3.0.4 version. 
>Currently a subnet is split into half with each DHCP server 
>allocating IP from its own half. It is sort of a primitive load-
>Now I intend to move both servers to DHCP failover mechanism with 
>shared and the complete subnet. Everything works during the testing, 
>however my goal is to do the migration to failover by preserving 
>existing leases. I don't want to delete lease database before 
>enabling the failover feature on both servers. Is it possible? If so, 
>how it should be done?

You can combine the two lease databases by stopping both servers, and 
appending one lease file to the other. Configure one server to 
allocate addresses from the sum of the two previous ranges, and 
restart just the one - you now have one server with a record of all 
the leases. When you add failover, the combined database will be 
copied to the other server.

Note that clients with an existing lease will initially attempt to 
contact ONLY the server that issued the lease. At some point they 
will then fallback to broadcasting, at which point the remaining 
server can answer and offer their previous address. This will only 
affect the clients with addresses from the server that is taken out 
of service during this process.

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