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    DHCP Auto Logout Overview

    This topic provides an introduction to the optional Dynamic Host Configuration Protocol (DHCP) auto logout feature and includes the following sections:

    Auto Logout Overview

    Auto logout is an optional configuration for a DHCP local server that improves the efficiency of DHCP IP address assignment. Auto logout enables IP addresses to be immediately released and returned to the address pool when the addresses are no longer used by DHCP clients. DHCP can then assign the addresses to other clients. Without auto logout, an IP address is blocked for the entire lease period, and DHCP must wait until the address lease time expires before reusing the address.

    Auto logout is particularly useful when DHCP uses long lease times for IP address assignments and to help avoid allocating duplicate IP addresses for a single client. For example, you might have an environment that includes set-top boxes (STBs) that are often upgraded or replaced. Each time an STB is changed, the new STB repeats the DHCP discover process to obtain client configuration information and an IP address. DHCP views the new STB as a completely new client and assigns a new IP address—the previous IP address assigned to the client (the old STB) remains blocked and unavailable until the lease expires. If auto logout is configured in this situation, DHCP recognizes that the new STB is actually the same client and then immediately releases the original IP address.

    How DHCP Identifies and Releases Clients

    The auto logout feature requires that DHCP explicitly identify clients. By default, the DHCP local server identifies clients based on the MAC address or client identifier. However, in some cases, this type of identification might not be sufficient. For example, in the previous STB example, each STB has a different MAC address, so DHCP incorrectly assumes that an upgraded or replacement STB is a new client.

    To explicitly identify clients, auto logout uses a secondary identification method when the primary identification method is unsuccessful—the primary method is considered unsuccessful if the MAC address or client identifier does not match that of an existing client. The secondary identification method is based on the DHCP option 60 and option 82 information in DHCP discover messages.

    Both the primary and secondary identification methods use subnet information to differentiate between clients. The primary identification method differentiates between two clients with the same MAC address (or the same client identifier) if the clients are on different subnets. Similarly, the secondary identification method considers two clients as different if they have the same option 60 and option 82 information, but different subnets.

    The DHCP local server immediately releases the existing address when auto logout is enabled and the secondary identification method identifies a duplicate client (that is, the discover packet is from an existing client).

    Option 60 and Option 82 Requirements

    The DHCP local server requires that the received discover packet include both DHCP option 60 and option 82. If either option is missing, the DHCP local server cannot perform the secondary identification method and auto logout is not used.

    Modified: 2017-08-31