Failover : Balancing a one-ip pool (!)

Nicolas Ecarnot nicolas at
Tue Feb 16 15:46:45 UTC 2016

Thank you Chris for your answers.

See below some comments.

Le 16/02/2016 15:28, Chris Buxton a écrit :
> Nicolas,
> I have a couple of thoughts about this.
> 1. When using a hub-and-spoke architecture, I find its better to let
> the spokes be primary and the hub be secondary for all its peers.

We lived this way (spokes primary and hub secondary) for 6 years.
Actually, we saw no real difference, neither in term of reliability not 

> 2. Singleton pools like you're describing don't really work for the
> failover protocol. Only the primary server will ever be able to
> answer. You won't see any benefit in terms of load balancing or even
> fault tolerance.

Though I understand your answer about load balancing, I counldn't agree 
about fault tolerance benefit:
On every remote site, we configure its router to relay the queries 
towards 2 servers. When one of the server is down, the other correctly 
replies, even to one-ip pools queries.

> You're better off configuring these without
> failover; if you really can't use reservations (host declarations
> with fixed addresses), then just define each singleton pool
> identically on each server, without failover (as long as you're
> certain there will never be two clients asking for that one
> address).

I agree, that could make complete sense, and would be cleaner.

> Regards, Chris
>> On Feb 16, 2016, at 12:05 AM, Nicolas Ecarnot <nicolas at>
>> wrote:
>> Hello,
>> This message is less a question than a discussion, because we're
>> happy with our setup for years and maybe won't change soon, unless
>> you answer with a nice new idea.
>> We have 7 DHCP servers in a star pattern : one central and 6
>> around. The center is the primary peer for the others, for around
>> 400 pools and lots of subnets.
>> Around 30 pools are classical ones, and balancing fine. The
>> numerous other ones are one-ip pools.
>> These one-ip pools are made this way in order to : - benefit from
>> failover setup. There seems to be no other way, failover can only
>> be setup on pools - apply classID rules by pool, working only on
>> one machine by subnet. We don't want to use static reservations for
>> that.
>> All this is working like a charm, but a small concern of mine is
>> the periodic balancing (60 seconds by default, we raised it to 120)
>> where I see that the servers are trying and obviously failing to
>> balance the only one ip between peers. This is not very bad, except
>> it is eating CPU and flooding the logs.
>> But I post here this message in hope someone could advice a better
>> way to do. Because I read lots of doc about balancing and the way
>> ISC DHCPD is managing this sounds clever and I'd be happy to use it
>> in a proper way.
>> Of course, external constraints exist forcing me to use this weird
>> one-ip pools setup. But I'll fight to change them if someone
>> replies with nice hints I may have missed.
>> Regards,
>> -- Nicolas ECARNOT _______________________________________________
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