Failover and config differences

[Phil Mayers]

All,

We run DHCP with a mix of fixed-address statements for desktop PCs, and 
dynamic pools for roaming laptops and wi-fi clients. The config - 
subnets, host and pool statements - is generated out of our SQL database 
every 15 minutes and diff/reload. There are tens of thousands of each 
type of IP (fixed-address and pool).

Many moons ago, we used to run DHCP failover between two servers, but we 
abandoned it during a re-architect of our config generation in favour of 
"split the lease pool on half". At that time, this was sufficient (and 
simpler).

As time has gone by, the usage of the dynamic pools has risen to the 
point where one server is no longer enough to serve the number of 
clients - particularly on our wireless network. For this reason we're 
considering re-enabling failover, but I have a couple of concerns.

One is that, as noted, we re-build and reload the DHCP config quite 
frequently. 20-30 times a day might not be unusual, depending on the 
number of updates into the database.

I was never clear how (if at all) failover handled the config being 
different at each end. For example, suppose I have a config fragment:

network x netmask y {
   pool {
     192.168.1.10 192.168.1.20;
     failover peer xxx;
   }
}

...and the pool changes for whatever reason:

network x netmask y {
   pool {
     192.168.1.10 192.168.1.18;
     # 192.168.1.19 is now gone
     192.168.1.20;
     failover peer xxx;
   }
}

...and one DHCP server re-starts fractionally (say a minute, for 
example) before the other. What happens?

Related: what happens if I make an OMAPI update that affects lease 
allocation, and the updates don't happen on both servers at the same time?

Also related: if we have an extended outage and I put the server into 
partner-down, can I continue to update the config on the working server, 
or should I stop the updates? Or do I need to guarantee in some 
unspecified fashion that the dead server can't re-animate until it has 
the newest config?

Cheers,
Phil