Failover question

Simon Hobson dhcp1 at
Fri Oct 6 18:30:00 UTC 2006

Lars Jacobsen wrote:

>Planning to run a failover setup and reading the man pages I saw there
>need to be a "split statement" defining witch server responds to the
>But does this also mean that the servers "share" the pool between them
>and keeps each other informed/in sync about witch leases they have
>handed out?

They 'share' the free addresses, in that there is a balancing 
algorithm that will 'transfer ownership' of free address should one 
server end up with significantly more than the other. During normal 
operation, the servers exchange details of the leases issued so that 
both of them have the full picture. If one fails (and you put the 
other in partner down state) then the remaining server can answer 
renewal requests for any of the leases. During normal operations, the 
client will renew with the server that gave it it's current lease.

>This I because I plan to have a pools down to just 1 IP address and thus
>if the secondary has handed out the lease how does it show on the

I don't believe it works well with only one address in a pool - it's 
not possible to balance zero or one free lease !

What are you trying to achieve, there may be better ways of doing it.

>Links to state transition diagrams and descriptions on failover setups
>are appreciated to clear my understanding.

"The DHCP Handbook" by Ralph Droms and Ted Lemon is considered by 
some to be the definitive work. Make sure you get the second edition 
which was (I understand, I only have the first edition) expanded to 
include failover.

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