Many factors must be considered when you decide how long to set your client leases. Consider the following:
Another important consideration is that clients attempt to renew their leases half-way through the lease duration. The longer the lease, the longer it takes for client configuration changes to be registered with the DHCP server. It also takes longer for the server to realize that a previously assigned address is no longer in use.
Another issue to consider concerns outages and access to the DHCP server. If a client loses access to its DHCP server before renewing its lease, it must stop using the network after the lease expires. If a client is turned on and connected to the network at the time of the outage, however, the lease does not expire.
The longest lease provided by a DHCP server determines the length of time you might have to wait before configuration changes can be propagated within a network. This length of time could mean manually restarting every client or waiting the amount of time required for all leases to be renewed before the changes take effect. If your site policy is to turn workstation power off at the end of the day, clients could acquire configuration changes at least once per day.
NOTE: All lease considerations refer to DHCP clients or devices only. For clients or devices that use BOOTP, you must bring down the device and restart it to acquire any new configuration changes.
For more information, see:
When considering the length of leases, ask these questions:
The default of three days provides a good balance between a long-lease and a short-lease duration.
If you have more users than IP addresses, keep leases short to allow access to more users. A short lease could be 15 to 30 minutes, two to four hours, or even a matter of days.
If your site's usage pattern shows that all clients request an address every day and you have half as many addresses as users, lease times in hours or minutes would provide access to more users.
If your site has mobile users or provides remote access to clients, plan to provide service for these clients on a specific subnet. Providing support, including special options the clients might require, makes network administration of the clients easier.
If your site's usage pattern indicates that your users typically use an address for only one or two hours, that should be your minimum lease time.
Shorter leases support more clients, but shorter leases also increase the load on the DHCP server and network bandwidth. A lease of two hours is long enough to serve most users, and the network load should be negligible. A lease of one hour or less might increase network load to a point that requires attention.
By locating a DHCP server in close proximity to its users, the network load should be negligible over LAN connections. If a DHCP server must communicate over WAN links to provide service to clients, slowdowns and time-outs might occur.
If your typical server outage lasts two hours, a lease of four hours would avoid loss of lease to clients that were active at the time of the server outage.
We recommend setting your lease times to twice the length of a typical server outage.
The same recommendation applies to communications line outages. If a communications line is down long enough that leases expire, you might see a significant network load when the service is restored.
If you have users who require a lease for important job functions, consider lease times for them that are twice the length of a maximum server outage. For example, if your DHCP server were to go down on Friday evening and require the entire workday Monday to be restored, that would be an outage of three days. In this case, a six-day lease covers that situation.
If you have users setting up Web pages or archiving data for others to access, they want addresses that do not change. You might want to assign permanent addresses for these users instead of assigning long lease times (three weeks or two months, for example).
The relevant length of time is the maximum amount of time you want to allow a client to keep an address, even if the host computer is turned off. For example, if an employee takes a four-week vacation and you want the employee to keep his or her address, a lease of eight weeks or longer is required.
Table lists examples of lease times and reasons why these times were chosen.
Table . Lease Time Examples
There usually is a trade-off when an attempt is made to control specific client access to leases. Typically, you would manually configure each client and dedicate an IP address permanently to each client. Novell's DHCP server, however, provides control based on the client's hardware address.