DHCP Dimensioning Guidelines

[Simon Hobson]

Aggarwal Vivek-Q4997C wrote:

>Im having a network 20 K Subscribers. I want to install the ISC DHCP
>Server 3.0.6 for 20 K Subs.
>
>Can I anyone help me in dimensioning the DHCP Server, in terms of Hard
>Disk and RAM (Memory) required. Also the guidelines on basis of which
>one will dimension the DHCP Server

If you look back in the archives you'll see something similar comes 
up fairly regularly - usually phrased in terms of "how many users ..."

The answer is that there are no hard an fast rules because there are 
far too many variables. The first one is the lease length you intend 
to use - 1 hour means approx 40k renewals/hr, two weeks means approx 
3k (20k/7) renewals/day or around 125/hr.

After that is type of client - are these 'always on' routers, turn on 
between 8am and 9am business PCs, or turn on at random home users ? 
'Always on' devices will only need to renew at half the lease time 
(by default) so making long leases will reduce the renewal rate - but 
if there are 20k business user who all turn on between 8 and 9am then 
that's all 20k users to service in one hour. Also, PCs will tend to 
request a lease extension, consumer routers will start with a 
discover (which is effectively 2 transactions - discover-offer and 
request-ack).

Will you be doing DNS updates - that has a big impact on the work 
required to issue 7 expire leases (but not on renewals).

Do you have any reasonable expectation of a wide outage ? Ie, is 
there any chance of all 20k (or a significant proportion of them) 
trying to boot at once ?

What about the rest of your infrastructure ? Will devices need to 
boot off a server - which would of course require (eg) a TFTP server 
capable of handling clients at the same rate.

If you expect the possibility (although fairly remote) and having a 
large number of clients boot at once, can you do anything to mitigate 
the demand. Eg, if you are an ISP serving a whole continent and wish 
to deal with the remote possibility of a widespread power outage, can 
you keep part of the network shut down for a while - and bring more 
online as the load subsides ?



Having said all that, the biggest bottlenecks on most large DHCP 
servers are the disks and DNS updates. For a large installation you 
may want to consider keeping the lease database on battery backed ram 
disk for performance. This area is critical as EVERY transaction 
results in a write - to meet the requirement that leases are recorded 
to permanent storage BEFORE being offered to clients (incidentally 
one of the things the MS server does NOT do). Logging also has an 
effect, but you can reduce it by setting the logging to async mode.