DHCP Overview

DHCP is based on BOOTP, a bootstrap protocol that allows a client to discover its own IP address, the IP address of a server host, and the name of a bootstrap file. DHCP servers can handle requests from BOOTP clients, but provide additional capabilities beyond BOOTP, such as the automatic allocation of reusable IP addresses and additional configuration options.

Note: You cannot configure the Services Router as both a DHCP server and a BOOTP relay agent.

DHCP provides two primary functions:

• Allocate temporary or permanent IP addresses to clients.
• Store, manage, and provide client configuration parameters.

As a DHCP server, a Services Router can provide temporary IP addresses from an IP address pool to all clients on a specified subnet, a process known as dynamic binding. Services Routers can also perform static binding, assigning permanent IP addresses to specific clients based on their media access control (MAC) addresses. Static bindings take precedence over dynamic bindings.

DHCP Options

In addition to its primary DHCP functions, you can also configure the Services Router to send configuration settings like the following to clients through DHCP:

• IP address of the DHCP server (Services Router).
• List of Domain Name System (DNS) and NetBIOS servers
• List of gateway routers
• IP address of the boot server and the filename of the boot file to use
• DHCP options defined in RFC 2132, DHCP Options and BOOTP Vendor Extensions

Compatibility with Autoinstallation

Services Router DHCP server functions are compatible with the autoinstallation feature. The DHCP server automatically checks any autoinstallation settings for conflicts and gives the autoinstallation settings priority over corresponding DHCP settings. For example, an IP address set by autoinstallation takes precedence over an IP address set by the DHCP server.

(To configure autoinstallation, see Configuring Autoinstallation.)

Conflict Detection and Resolution

A client that receives an IP address from the Services Router operating as a DHCP server performs a series of Address Resolution Protocol (ARP) tests to verify that the address is available and no conflicts exist. If the client detects an address conflict, it informs the DHCP server about the conflict and can request another IP address from the DHCP server.

The Services Router maintains a log of all client-detected conflicts and removes addresses with conflicts from the DHCP address pool. To display the conflicts list, you use the show system services dhcp conflict command. The addresses in the conflicts list remain excluded until you use the clear system services dhcp conflict command to manually clear the list.

Interface Restrictions

The Services Router supports DHCP client requests received on Fast Ethernet interfaces only. However, DHCP requests received from a relay agent are supported on all interface types.

DHCP is not supported on interfaces that are part of a virtual private network (VPN).