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DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers
(Page 2 of 2)
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.
 Key Concept: DHCP leases follow a conceptual life cycle. The lease is first assigned to the client through a process of allocation; if the device later reboots it will reallocate the lease. After a period of time controlled by the renewal timer (T1), the device will attempt to renew its lease with the server that allocated it. If this fails, the rebinding timer (T2) will go off and the device will attempt to rebind the lease with any available server. The client may also release its IP address if it no longer needs it.
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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.
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