Re: Choosing length of lease

["Michael J. Lewis" <[e-mail]>]

John M. Wobus wrote:
> 
> Thanks to all who have answered questions I've posed here for the
> purposes of getting material for the DHCP FAQ & thanks to everyone on
> the list for humoring my queries.
> 
> Another question I'd like to pose and answer in the DHCP FAQ is "How
> long should a lease be?" In the interest in offering something more
> helpful than "it depends on your site/goals/etc",  I pose these
> questions:
> 
> (1) At your own site, how long do you make leases and why did you choose
>     this time?
> 
> (2) Are there lease-lengths that you would consider to be a lower-bound
>     or upper-bound for any ALL practical uses?  If so, what would you
>     be trying to avoid by staying above/below this bound.
> 
Our lease recommendations are a bit convoluted.  We originally started
with a long lease (60 days - based on the amount of time we felt an IP
address was truly no longer in use) but found this totally unwieldy,
particularly at our sites with high concentrations of mobile users.  We
now no longer recommend a maximum lease but do recommend a minimum lease
of 6 days.  This length allows a site to recover from a worst case
scenario where a DHCP server crashed on a Friday evening and was not
discovered until Monday morning.  With a six day lease, support has
about 24 hours to recover the server prior to any existing leases being
lost.  The 6 day lease covers a normal weekend; 8 or 10 days would be
needed for 4 and 5 days weekends respectively.

Despite our recommendations, many sites employ 3 days a lease time
because that is the default with Microsoft's server.  One site is using
2 days as they are trying to fit about 280 requests into a subnet of 
250 addresses.  Most of the other sites probably fit into the 14 day
range as this has the effect of practically moving all DHCP traffic 
out of the subnet outside of the Monday morning reboot.