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Extended DHCP Local Server Overview

You can enable a router to function as an extended Dynamic Host Configuration Protocol (DHCP) local server and configure the extended DHCP local server options on the router. The extended DHCP local server provides an IP address and other configuration information in response to a client request.

The extended DHCP local server enhances traditional DHCP server operation by utilizing centralized address-assignment pools. The address-assignment pools are managed independently of the DHCP local server and can be shared by different client applications.

Note: You cannot configure the extended DHCP local server, DHCP client and extended DHCP relay on the same interface.

To configure the extended DHCP local server on the router, you include the dhcp-local-server statement at the [edit system services] hierarchy level.

To configure the extended DHCP local server in a routing instance, include the dhcp-local-server statement at the [edit routing-instances routing-instance-name system services] hierarchy level.

To configure the extended DHCPv6 server on the router, you include the dhcpv6 statement at the [edit system services dhcp-local-server] hierarchy level.

To configure the extended DHCPv6 server in a routing instance, include the dhcpv6 statement at the [edit routing-instances routing-instance-name system services dhcp-local-server] hierarchy level.

Interaction Among the DHCP Client, Extended DHCP Local Server, and Address-Assignment Pools

In a typical carrier edge network configuration, the DHCP client is on the subscriber’s computer, and the DHCP local server is configured on the router. The following steps provide a high-level description of the interaction among the extended DHCP local server, DHCP client, and address-assignment pools:

1. The DHCP client sends a discover packet to one or more DHCP local servers in the network to obtain configuration parameters and an IP address for the subscriber.
2. Each DHCP local server that receives the discover packet then searches its address-assignment pool for the client address and configuration options. Each local server creates an entry in its internal client table to keep track of the client state, then sends a DHCP offer packet to the client.
3. On receipt of the offer packet, the DHCP client selects the DHCP local server from which to obtain configuration information and sends a request packet indicating the DHCP local server selected to grant the address and configuration information.
4. The selected DHCP local server sends an acknowledgment packet to the client that contains the client address lease and configuration parameters. The server also installs the host route and Address Resolution Protocol (ARP) entry, and then monitors the lease state.

Minimal Configuration for Clients

The extended DHCP local server provides a minimal configuration to the DHCP client if the client does not have DHCP option 55 configured. The server provides the subnet mask of the address-assignment pool that is selected for the client. In addition to the subnet mask, the server provides the following values to the client if the information is configured in the selected address-assignment pool:

• router—A router located on the client’s subnet. This statement is the equivalent of DHCP option 3.
• domain name—The name of the domain in which the client searches for a DHCP server host. This is the default domain name that is appended to hostnames that are not fully qualified. This is equivalent to DHCP option 15.
• domain name server—A Domain Name System (DNS) name server that is available to the client to resolve hostname-to-client mappings. This is equivalent to DHCP option 6.

DHCP Local Server and Address-Assignment Pools

In the traditional DHCP server operation, the client address pool and client configuration information reside on the DHCP server. With the extended DHCP local server, the client address and configuration information reside in centralized address-assignment pools, which are managed independently of the DHCP local server and which can be shared by different client applications.

The extended DHCP local server also supports advanced pool matching and the use of named address ranges. You can also configure the local server to use DHCP option 82 information in the client PDU to determine which named address range to use for a particular client. The client configuration information, which is configured in the address-assignment pool, includes user-defined options, such as boot server, grace period, and lease time.

Configuring the DHCP environment that includes the extended DHCP local server requires two independent configuration operations, which you can complete in any order. In one operation, you configure the extended DHCP local server on the router and specify how the extended DHCP local server determines which address-assignment pool to use. In the other operation, you configure the address-assignment pools used by the extended DHCP local server. The address-assignment pools contain the IP addresses, named address ranges, and configuration information for DHCP clients. See Configuring Address-Assignment Pools for details about creating and using address-assignment pools.

Note: The extended DHCP local server and the address-assignment pools used by the server must be configured in the same routing instance.

Note: You can apply DHCP configurations on Integrated Routing and Bridging (IRB) interfaces.