Max number of subnets DHCP can handle

[Chris Fabri]

I've recently been asked to set up DHCP to handle 3840 subnets.  I've 
run into a lot of problems with failover handling this.     I found a 
thread:

http://marc.info/?l=dhcp-users&m=119203926819005&w=2

That covers handling a large number of leases, where there are about 150 
subnets, but they are large subnets (/19s-/21s).   My situation is the 
inverse of this, they are all /24s.

Currently testing on Intel 1.3ghz, 512MB ram, SATA drives.    I've tried 
versions 3.0.1 (RedHat 4s RPM) and 3.1.0 ISC source, with no mods.   In 
standalone, 3.0.1 takes a while to load, 3.1.0 loads quickly.    But 
when in failover, both DHCP processes start, but the primary appears to 
simply spend all it's time calculating lease pool balance - the log just 
scrolls through constantly with pool balance numbers, the secondary 
server eventually gets a failover timeout.    The secondary will answer 
and fulfill requests, but the primary doesn't.

In failover in 3.1.0, it also tries to pre-populate the leases file, but 
it only gets through a couple subnets.    3.0.1 doesn't do this.    I 
did test v3.0.1 with this large config, and with a lease file containing 
about 57K leases, and it loads ok in standalone config, I haven't 
bothered trying this in a failover situation, as I don't even get that far.

It appears from the thread above there is some tweaking for handling a 
large number of leases.  That 57K leases file I got from our production, 
failover DHCP server, which has 826 subnets.   The situation where I 
have 3840 subnets isn't likely to have more leases, and probably will 
have less.   I guess I'm looking for either some way to tweak the config 
to deal with my situation, or an idea of what number of subnets is going 
to cause the problem.   Load time is a concern, right now it's about 2 
minutes on this hardware, and obviously startup time affects failover to 
some degree, but getting failover working is the bigger issue.

I'm trying to find out if this number of subnets is really necessary, 
I'm not privy to all the details with the project that requires this.

Any input is greatly appreciated, hopefully I've provided enough 
details, but I'm sure I've left out something important.   chris