Failover and config differences

[Chris Buxton]

On Dec 12, 2012, at 4:28 AM, Phil Mayers wrote:
> 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.

[...]

> 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?

A couple of points:

1. Failover does not give significantly better performance than one server alone, or two servers using split scopes (as you currently have). In my experience, depending on a number of factors including configuration, performance might be +/- 30% or so compared to a single stand-alone server. With split scopes, you're likely to get better performance because the two servers aren't constantly notifying each other of lease changes.

2. When using failover, if a server has been down for a while, you should make sure the configs are synchronized before starting it back up again. (If you don't, you're just going to have to stop and restart it again after you do synchronize the changes.) After it starts up, it synchronizes with its peer before starting to answer requests.

Chris Buxton
BlueCat Networks