The TCP/IP Guide - DHCP Lease Renewal and Rebinding Processes

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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 Configuration and Operation

DHCP Lease Reallocation Process

DHCP Early Lease Termination (Release) Process

DHCP Lease Renewal and Rebinding Processes
(Page 3 of 3)

The Purpose of the Two-Step Rebinding/Renewal Process

One valid question is: why bother with a two-step process, rebinding and renewal? The reason is that this provides the best blend of efficiency and flexibility. We first try to contact the server that granted the lease using a unicast request, to avoid taking up the time of other DHCP servers and disrupting the network as a whole with broadcast traffic. Usually this will work, because DHCP servers don't change that often and are usually left on continuously. If that fails, we then fall back on the broadcast, giving other servers a chance to take over the client’s existing lease.

Key Concept: Each client’s lease has associated with it a renewal timer (T1), normally set to 50% of the length of the lease, and a rebinding timer (T2), usually 87.5% of the lease length. When the T1 timer goes off the client will try to renew its lease by contacting the server that originally granted it. If the client cannot renew the lease by the time the T2 timer expires, it will broadcast a rebinding request to any available server. If the lease is not renewed or rebound by the time the lease expires, the client must start the lease allocation process over again.

DHCP Lease Reallocation Process

DHCP Early Lease Termination (Release) Process

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Version 3.0 - Version Date: September 20, 2005

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