performance issues with 4096 subnets

Simon Hobson dhcp1 at
Fri Aug 10 10:55:28 UTC 2012

<Campbell.ColinD at> wrote:

>We have a requirement to subdivide:
>a) into 32k subnets, each a /24 with 4 pools in each subnet
>b) into 4k subnets, each a /21 with 1 pool in each subnet
>So far we've only attempted (b). The results are less than impressive.

How big are the pools ?
If you have large pools (say 200 addresses each in the /24 subnets 
and 2000 in each of the /21 subnets), then that's  13 to 14 million 
addresses that need to be hashed and memory structures created for.
Yes, that will take time to process, and memory to hold.
If networks don't need that many dynamic addresses, then cutting back 
on the pool sizes will make a big difference to performance.

The others have given some good advice.
On top of that, make sure your server has enough memory - paging will 
kill performance.
Put the leases file on fast disk. Every lease operation results in a 
file write & sync to the leases file, and on a busy system this can 
be a bottleneck.

I'm guessing from your email address and the numbers involved that 
this is something along the lines of "subnet per police station or 
regional/divisional office" and all handled from one location. That's 
going to put an awful lot of dependency on a single server ! If you 
go failover, then your performance issues increase as there's the 
overhead of the failover stuff to consider.

Apart from the problems of a site disconnected (so it loses DHCP as 
well as access to central systems), you may want to consider the 
result of a widespread issue - for example a widespread network or 
power failure. The latter is the biggest issue - can your serve cope 
with those millions of devices all powering up within a few minutes ? 
It happens ! Parts of the USA have had it, half on India have just 
had it, ...
It's not just a case of "devices will just have to queue". If the 
server can't cope, then devices will time out and retry. This means 
the load on the server goes even higher, so it takes longer to 
respond to any client, so they'll have sent more packets in the 
meantime, which puts the load up and response times up, so the 
clients will have waited even longer and sent even more packets, and 

Simon Hobson

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