How Dhcp Identifies And Releases Clients; Option 60 And Option 82 Requirements - Juniper ACX1000 Configuration Manual

ACX Series Universal Access Router Configuration Guide

How DHCP Identifies and Releases Clients

Option 60 and Option 82 Requirements

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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.
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).
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.
Extended DHCP Local Server Overview on page 315
DHCP Local Server Handling of Client Information Request Messages on page 335
DHCP Duplicate Client Differentiation Using Client Subinterface Overview on page 362
Address-Assignment Pools Overview on page 317
Automatically Logging Out DHCP Clients on page 335
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