DHCP Failover and Performance

[John Wobus]

On Feb 22, 2012, at 3:31 AM, Smith Bill wrote:
> Hi
>
> I have recently set up a proof of concept system and configured DHCP  
> for failover; one of my initial observations is that configuring  
> failover seems to have a dramatic effect upon the time taken to  
> issue an IP Address.
>
> Does anyone have any performance data for ISC DHCP that they could  
> share please?

What is "dramatic" in this case?  In my own case, several years ago,  
we didn't
notice any performance degradation when we switched to a redundant  
configuration,
only that it was fast enough for us both before and after, for a  
system serving
thousands of simultaneous one-hour leases.

If some clients are unable to reach one of the DHCP servers (including,
if relevant, working helper address) then half those clients will
experience a short delay, by design of the ISC server's load balancing  
method.

Also, FT configurations begin sessions with a short lease, thus  
generating
more work for the server.  It doesn't seem likely, but if your load was
very close to your system's capacity, conceivably that could push you  
over.
The shorter lease time is under control of the configured lease time AND
the parameters set in the FT setup.  I don't recall details but the man
pages and the DHCP book probably explain it.

The other answers I've seen on list to this question address ways to
maximize the throughput of an ISC server, which could conceivably
help in your case, but aren't specifically tied to a FT setup
unless your load was already pretty close to high enough to degrade
system performance.

John Wobus
Cornell