Supported Platforms
Related Documentation
DHCP Auto Logout Overview
This topic provides an introduction to the optional DHCP auto logout feature and includes the following sections:
Auto Logout Overview
Auto logout is an optional configuration for DHCP local server and DHCP relay agent 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 (STB) that are often upgraded or replaced. Each time a 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. DHCP relay agent acts as a proxy client for auto logout and sends a DHCP release message to the DHCP server.
How DHCP Identifies and Releases Clients
The auto logout feature requires that DHCP explicitly identify clients. By default, DHCP local server and DHCP relay agent identify clients based on 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.
In order 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 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.
DHCP local server and DHCP relay agent perform the following operations when auto logout is enabled and the secondary identification method identifies a duplicate client (that is, the discover packet is from an existing client).
- DHCP local server immediately releases the existing address.
- DHCP relay agent immediately releases the existing client
and then sends a DHCP release packet to the DHCP server. Sending the
release packet ensures that DHCP relay and the DHCP server are synchronized.
If the DHCP relay receives a DISCOVER message from an existing client, the DHCP relay forwards the DISCOVER message to the DHCP server. The DHCP relay preserves the binding if the client's existing IP address is returned by the DHCP server. This behavior is not applicable if the proxy-mode override or client-discover-match functionality are enabled.
Note: If the DHCP relay agent is in snoop mode, DHCP relay releases the client but does not send a release packet to the DHCP server if the discover packet is for a passive client (a client added as a result of snooped packets) or if the discover packet is a snooped packet.
Option 60 and Option 82 Requirements
DHCP local server requires that the received discover packet include both DHCP option 60 and option 82. If either option is missing, DHCP local server cannot perform the secondary identification method and auto logout is not used.
DHCP relay agent requires that the received discover packet contain DHCP option 60. DHCP relay determines the option 82 value based on the guidelines provided in DHCP Relay Agent Option 82 Value for Auto Logout.
