Configuring lease time
What should be my lease time or How do I select my lease time?
This is very common debate that I keep having with my collogues and I am sure this question must be haunting many dhcp admin’s mind as well. Lease time can be as low as 15 minutes( tech. it can be in order of secs) to INFINITE. However there are few parameters that an admin must keep in mind while deciding on the lease time. Those parameters are mostly the following
1. User load or density or number of user to support vs. number of free Ip address
2. User affinity to the site
3. Mobility,
4. frequency of network configuration changes
There may be additional parameters which can be added but I think considering the above will solve most of your issue wrt lease time
1. User load or density or number of user to support vs. number of available IP address
If the user load is high i.e. you have more number of users as compared to number of free ip address than to optimize the usage of Ip address you should keep the lease time short. May be in order of 1’s of hour. Recommended is 4 or 8 hours. If you have more number of free ip address then the number of users to support then you can have a long lease time in order of Days. Microsoft default is 8 days. You can safely use it
2. User affinity to the site
In public location like coffer shop, airport, railway station, etc. where user keep visiting the lease time should be short like 15 min. to avoid unnecessary blockage of ip address by stale user. By doing this you will make sure that at given point of time you have minimum blocked ip address that are not being used by active users and thus ensure optimum usage of your ip address scope
In offices where user location is permanent eg. Desktop it’s recommended to have a long lease time like Microsoft dhcp server default 8 days.
3. Mobility
If you need to support mobile users carrying notebooks , hand held devices ,etc then you should have short lease time in order of hours like 2 hrs or 4 hrs or 8 hrs
4. frequency of network configuration changes
If the network configuration change happens quite frequently may be weekly or monthly then in that case its better to keep the lease time in order of 12 hrs like 12 hr or 1 day. As longer the lease time longer it takes for client configuration changes controlled by DHCP server to propagate.
Should I have INFINITE lease time?
It’s technically possible and most of the devices support it but the recommendation is never to have infinite lease time. The main reason is that any change in network configuration on dhcp server will not be updated on the client as the client will not trigger renew. Also it’s reported on some site that few devices don’t behave properly with INFINITE lease time and result in service crash and other issues. So if you are dhcp admin and want to avoid unnecessary issues it’s recommended not to have INFINITE lease time
Neelmani
Windows Enterprise Networking
[This posting is provided "AS IS" with no warranties, and confers no rights.]