Defining DHCP Lease Periods

Problem

You want to change the default lease time.

Solution

To modify the default DHCP lease time for a pool of IP addresses, use the lease configuration command:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#ip dhcp pool 172.25.2.0/24
Router1(dhcp-config)#lease 2 12 30 
Router1(dhcp-config)#exit
Router1(config)#end
Router1#

Discussion

The lease command takes up to three options: lease days [hours] [minutes], with hours and minutes being optional. You can specify a maximum period of 365 days, 23 hours and 59 minutes, and a minimum of 1 second. The default is one day.

The shorter the lease period, the faster you can reconfigure DHCP options that may need to change. Short lease periods also permit IP addresses to be returned to the address pool for reallocation more quickly. This can be useful in environments where a large number of end devices connect and disconnect frequently, as in public wireless networks, such as at an airport. A short lease period of say 30 minutes might be useful to ensure that IP addresses are returned quickly to the shared pool. However, short lease periods also mean that workstations must renew their leases more often, which puts an extra strain on the network and DHCP server.

Conversely, a small office with a stable workforce may choose to increase their lease periods. Long lease periods can also reduce the impact of DHCP server failures. Unless a workstation reboots or needs to disconnect and reconnect to the network, most clients will wait until the lease is half expired before needing to talk to the server to renew it. If the server is unavailable, the client device will periodically retry the lease renewal until it succeeds. But most organizations have redundant DHCP servers, so there are few real benefits to extremely long lease periods.

In most situations, the default lease period of one day is sufficient. It allows the administrators to change global options in a timely fashion without putting an unnecessary burden on the network or server.

You can also configure the router to assign addresses with infinite lease periods by using the infinite keyword:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#ip dhcp pool COOKBOOK
Router1(dhcp-config)#lease infinite 
Router1(dhcp-config)#exit
Router1(config)#end
Router1#

Assigning an infinite lease period removes one of the major advantages of using DHCP. It can be extremely useful to be able to use DHCP to make wholesale configuration changes, but this means that the end devices have to check in periodically to renew their leases. Providing an indefinite lease period largely circumvents this advantage, since it forces you to wait until the end device disconnects and reconnects to the network before you can give it new information. Use the infinite keyword with caution.

You can view the lease expiration times of your active clients with the show ip dhcp binding command:

Router1#show ip dhcp binding 
IP address Hardware address Lease expiration Type
172.25.1.33 0100.0103.85e9.87 Infinite Manual
172.25.1.53 0100.0103.ea1b.ed Apr 11 2006 08:58 PM Automatic
172.25.1.57 0100.6047.6c41.a4 Apr 11 2006 09:17 PM Automatic
Router1#

Please refer to Recipe 20.7 for an explanation of the MAC address in this output.