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This feature module describes the Cisco IOS DHCP Relay Agent Support for Unnumbered Interfaces feature and includes the following sections:
Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently. Relay Agents receive Dynamic Host Configuration Protocol (DHCP) messages and then generate a new DHCP message to send out on another interface.
Previously, the Cisco IOS relay agent was not able to forward DHCP request messages if the interface on which the request arrived was IP unnumbered. The Cisco IOS DHCP relay agent now supports unnumbered interfaces. The DHCP relay agent automatically adds a static host route specifying the unnumbered interface as the outbound interface.
The Cisco IOS DHCP Relay Agent Support for Unnumbered Interfaces feature reduces configuration tasks and costs. In most cases, whenever an unnumbered interface is configured, a static route for any host beyond the unnumbered interface must be manually configured by a network administrator. For DHCP relay, this static route is automatically maintained.
The ability to support unnumbered interfaces provides the following benefits:
Stand.ards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
RFCs
For the Cisco IOS DHCP relay agent support for unnumbered interfaces functionality to work, the interface must be configured to be unnumbered with a helper address.
To use DHCP relay with an unnumbered interface, configure the interface with an IP helper-address pointing to the destination DHCP server.
In the following example, the DHCP relay receives a DHCP request on serial 0 and sends the request to the DHCP server located at IP helper-address 10.24.43.2. When it sends the DHCP request, the DHCP relay uses the IP address of Ethernet 0 for identification purposes and maintains this information for the life of the assigned IP address. This ensures that DHCP packets are sent to the proper DHCP server (10.24.43.2) and are returned to the client originator located at the other end of the unnumbered interface (serial 0, in this example):
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interface ethernet 0 ip address 10.108.6.6 255.255.255.0 ! interface serial 0 ip unnumbered ethernet 0 ip helper-address 10.24.43.2 !:
This functionality is automatically enabled if the interface is configured as unnumbered with an IP helper address.
None.
This section documents the ip dhcp database command, which was introduced in Cisco IOS Release 12.0(1)T. This optional command has been enhanced to allow the Cisco IOS DHCP relay agent to store the host route to the same remote file system as the DHCP server for recovery after reloads. All other commands used with this feature are documented in the Cisco IOS IP and IP Routing Command Reference, Release 12.1.
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Note The ip dhcp database command is not required for the DHCP relay support for unnumbered interfaces functionality. However, the command is necessary for the DHCP relay recovery capability. |
You can configure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) server and relay agent to save automatic bindings on a remote host called a database agent. To configure a DHCP server database agent and database agent parameters, use the ip dhcp database global configuration command. To remove the database agent, use the no form of this command.
ip dhcp database url [timeout seconds | write-delay seconds]
Syntax Description
url Specifies the remote file used to store the automatic bindings. Following are the acceptable URL file formats: timeout seconds (Optional) Specifies how long, in seconds, the DHCP server should wait before aborting a database transfer. Transfers that exceed the timeout period are aborted. By default, DHCP waits 300 seconds before aborting a database transfer. Infinity is defined as 0 seconds. write-delay seconds (Optional) Specifies how soon the DHCP server should send database updates. By default, DHCP waits 300 seconds (5 minutes) before sending database changes. The minimum delay is 60 seconds.
Defaults
DHCP waits 300 seconds for both a write delay and a timeout.
Command Modes
Global configuration
Command History
12.0(1)T This command was introduced.
Release
Modification
Usage Guidelines
The administrator may configure multiple database agents. Bindings are transferred using the File Transfer Protocol (FTP), Trivial File Transport Protocol (TFTP), or remote copy protocol (rcp).
Now the DHCP relay agent can save route information to the same database agents to ensure recovery after reloads.
Examples
The following example specifies the DHCP database transfer timeout value at 80 seconds:
ip dhcp database ftp://user:password@172.16.1.1/router-dhcp timeout 80
The following example specifies the DHCP database update delay value at 100 seconds:
ip dhcp database tftp://172.16.1.1/router-dhcp write-delay 100
Related Commands
show ip dhcp database Displays Cisco IOS DHCP server database agent information.
Command
Description
BOOTP--Bootstrap Protocol (BOOTP), RFC 951. Allows a booting computer to find out its IP address and the location of the boot file with the rest of the parameters.
client--A host trying to configure its interface (obtain an IP address) using DHCP or BOOTP protocols.
relay agent--A router that forwards DHCP and BOOTP messages between a server and a client on different subnets.
server--DHCP or BOOTP server.
Posted: Tue Aug 1 15:36:13 PDT 2000
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