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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Address Assignment and Dynamic Address Allocation and Management

DHCP Leases, Lease Length Policies and Management 1 2 DHCP Lease Address Pools, Ranges (Scopes) and Address Management

Renewal and Rebinding Timers

The processes of renewal and rebinding are designed to ensure that a client's lease can be extended before it is scheduled to end, so no loss of functionality or interruption occurs to the user of the client machine. Each time an address is allocated or reallocated, the client starts two timers that control the renewal and rebinding process:

• Renewal Timer (T1): This timer is set by default to 50% of the lease period. When it expires, the client will begin the process of renewing the lease. It is simply called “T1” in the DHCP standards.
  
  
• Rebinding Timer (T2): This timer is set by default to 87.5% of the length of the lease. When it expires, the client will try to rebind, as described above. It is given the snappy name “T2” in the DHCP standards.

Naturally, if the client successfully renews the lease when the T1 timer expires, this will result in a “fresh lease”, and both timers will be reset. T2 only comes into play if the renewal is not successful. It is possible to change the amount of time to which these timers are set, but obviously T1 must expire before T2, which must in turn expire before the lease itself ends. These usually are not changed from the default, but may be modified in certain circumstances.

The lease life cycle is described in the DHCP standards in the form of states that the client moves through as it acquires a lease, uses it, and then either renews or ends it. The next section on DHCP configuration describes these states and the specific exchanges of messages between a client and server to accomplish different lease activities.

DHCP Leases, Lease Length Policies and Management 1 2 DHCP Lease Address Pools, Ranges (Scopes) and Address Management