Default DHCP lease and CUCM 7.0 DHCP rebinding

Default DHCP Lease
Configuring the Address Lease Time

By default, each IP address assigned by a DHCP server comes with a one-day lease, which is the amount of time that the address is valid. To change the lease value for an IP address, use the following command in DHCP pool configuration mode:

Command	Purpose
Router(config-dhcp)# lease {days [hours][minutes] | infinite}	Specifies the duration of the lease. The default is a a one-day lease.

http://cisco.biz/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html#wp22915

CUCM 7.0 DHCP rebinding

Rebinding Timer (T2) Expires

If the client receives no reply from the server, it will remain in the RENEWING state, and will regularly retransmit the unicast DHCPREQUEST to the server. During this period of time, the client is still operating normally, from the perspective of its user. If no response from the server is received, eventually the rebinding timer (T2) expires. This will cause the client to transition to the REBINDING state, the T2 timer is set to 87.5% (7/8ths) (as per cisco’s recommendation it should be 75% of the lease time) of the length of the lease.

Client Sends DHCPREQUEST Rebinding Message

Having received no response from the server that initially granted the lease, the client “gives up” on that server and tries to contact any server that may be able to extend its existing lease. It creates a DHCPREQUEST message and puts its IP address in the CIAddr field, indicating clearly that it presently owns that address. It then broadcasts the request on the local network.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/7_1_2/ccmcfg/b02dhsrv.html