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Legacy DHCP and Extended DHCP

 

JDHCP or extended DHCP is the enhanced versions of the DHCP daemon available in the recent versions of Junos OS (non-EoL Junos releases). To find out the extended DHCP support for specific Junos OS release, see Feature Explorer.

Legacy DHCP functionality is deprecated—rather than immediately removed—to provide backward compatibility and an opportunity to bring your configuration into compliance with the new configuration.

Read this topic to understand the new enhancements and the changes done in CLI configuration statement syntax.

Understanding Differences Between Legacy DHCP and Extended DHCP

This topic covers the following sections:

New Features and Enhancements in Extended DHCP

Extended DHCP or JDHCP extends and enhances traditional DHCP operation. With the extended DHCP local server, the client configuration information resides in a centralized address-assignment pool, which supports advanced pool matching and address range selection. aAny new features are only added to the Extended DHCP. Extended DHCP supports following features and enhancements:

  • In extended DHCP, the address-assignment pools are external to the DHCP local server. The external address-assignment pools are managed by the authd process, independently of the DHCP local server, and can be shared by different client applications such as DHCP or PPPoE access. In legacy DHCP, client address pool and client configuration information reside on the DHCP server.

  • Extended DHCP server interacts with the local AAA Service Framework to use back-end authentication servers, such as RADIUS, to provide DHCP client authentication.

  • You can configure the dynamic profile and authentication support on a global basis or for a specific group of interfaces.

  • Extended DHCP local server supports IPv6 clients.

  • Both DHCP local server and DHCPv6 local server support the specific address request feature, which enables you to assign a particular address to a client.

  • 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.

  • You can 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.

  • The extended DHCP server supports following features:

Table 1 provides a comparison of the extended DHCP and a legacy DHCP configuration options.

Table 1: Comparing the Extended DHCP Local Server to the Traditional DHCP Local Server

Feature

Legacy DHCP Local Server

Extended DHCP Local Server

Local address pools

X

X

External, centrally-managed address pools

X

Local configuration

X

X

External configuration using information from address-assignment pools or RADIUS servers

X

Dynamic-profile attachment

X

RADIUS-based subscriber authentication, and configuration using RADIUS attributes and Juniper Networks VSAs

X

IPv6 client support

X

Default minimum client configuration

X

X

Benefits of Extended DHCP

  • Extended DHCP local server enhances traditional DHCP server operation by providing additional address assignment and client configuration functionality and flexibility in a subscriber-aware environment.

  • Extended DHCP local server enables service providers to take advantage of external address-assignment pools and integrated RADIUS-based configuration capabilities in addition to the continued support of traditional local address pools.

Change in Configuring DHCP Local Server in Extended DHCP Environment

In extended DHCP, use the following steps to configure DHCP server and address assignment pool:

  • Configure the extended DHCP local server on the device and specify how the DHCP local server determines which address-assignment pool to use.

  • Configure the address-assignment pools used by the DHCP local server. The address-assignment pools contain the IP addresses, named address ranges, and configuration information for DHCP clients.

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

Legacy DHCP and Extended DHCP Server Hierarchy Levels Changes

Legacy DHCP and extended DHCP servers can be configured at the hierarchy levels shown in Table 2:

Table 2: Legacy DHCP and Extended DHCP Server Hierarchy Levels

DHCP Service

Hierarchy

Legacy DHCP server

edit system services dhcp

Extended DHCP server

edit system services dhcp-local-server

Legacy DHCP relay

edit forwarding-options helpers bootp

Extended DHCP relay

edit forwarding-options dhcp-relay

Legacy DHCP address pool

edit system services dhcp pool

Extended DHCP address pool

edit access address-assignment pool

Since legacy DHCP is deprecated, that is, the commands are 'hidden' . These commands do not show in the help nor automatic completion. When you use the option show configuration to display your configuration, the system displays the following warning:

DHCP Statement Hierarchy and Inheritance

Junos OS devices support two syntax styles for configuring DHCP Client, Server, and Relay—for legacy DHCP and extended DHCP. Table 3, Table 4, and Table 5 provide differences in hierarchies for configuring some common features.

Table 3: DHCP Client Configuration - Difference in Legacy DHCP and Extended DHCP Server Hierarchy Levels

Legacy DHCP

Extended DHCP

Hierarchy Level:

[edit interfaces interface-name unit logical-unit-number family inet dhcp]

Hierarchy Level:

[edit interfaces interface-name unit logical-unit-number family inet dhcp-client]

client-identifier

  • ascii

  • hexadecimal

client-identifier

  • userid ascii

  • userid hexadecimal

Table 4: DHCP Server Configuration - Difference in Legacy DHCP and Extended DHCP Server Hierarchy Levels

Legacy DHCP

Extended DHCP

Hierarchy Level:

  • [edit system services dhcp]

  • [edit system services dhcp pool]

Hierarchy Level:

[edit access address-assignment pool pool-name family inet]

subnet-ip-address/mask

network

address-range

range

static-binding

  • mac-address

  • fixed-address

host host-name

  • hardware-address

  • ip-address

[edit system services dhcp pool subnet-ip-address/mask]

[edit access address-assignment pool pool-name family inet dhcp-attributes]

boot-file

boot-file

boot-server

boot-server

default-lease-time

maximum-lease-time

domain-name

domain-name

domain-search

option 119 string

exclude-address

excluded-address

maximum-lease-time

maximum-lease-time seconds

name-server

name-server

next-server

next-server

router

router

option

option

propagate-ppp-settings

propagate-ppp-settings

server-identifier

server-identifier

sip-server

  • address

  • name

sip-server

  • address

  • name

wins-server

wins-server

Hierarchy Level: [edit system services dhcp]

Hierarchy Level: [edit access address-assignment pool pool-name family inet]

option

option

byte-stream

hex-string

Table 5: DHCP Relay Configuration - Difference in Legacy DHCP and Extended DHCP Server Hierarchy Levels

Legacy DHCP

Extended DHCP

Hierarchy Level:

[edit forwarding-options helpers bootp]

Hierarchy Level:

[edit forwarding-options dhcp-relay]

dhcp-option-82

relay-option-82

interface interface-name

group group-name

relay-agent-option

relay-option-82

server

server-group

Note if you are using legacy DHCP—In legacy DHCP, DHCP configuration statements are organized hierarchically. Statements at the top of the hierarchy apply to the DHCP server and network, branches contain statements that apply to address pools in a subnetwork, and leaves contain statements that apply to static bindings for individual clients.

To minimize configuration changes, include common configuration statements shown in tables above. For example, include the domain-name statement at the highest applicable level of the hierarchy (network or subnetwork). Configuration statements at lower levels of the hierarchy override statements inherited from a higher level. For example, if a statement appears at both the [edit system services dhcp] and [edit system services dhcp pool] hierarchy levels, the value assigned to the statement at the [edit system services dhcp pool] level takes priority.

Difference in Legacy DHCP Relay and Extended DHCP Relay

Legacy DHCP Relay can work as a DHCP IP helper, forwarding DHCP packets from DHCP servers to all interfaces. Extended DHCP Relay cannot work as an DHCP IP helper; it can leverage Option-82 to forward DHCP packets from DHCP server. See DHCP Relay Agent Information Option (Option 82).

Restrictions in Using Legacy DHCP and Extended DHCP

Remember the following items while configuring extended DHCP:

  • You can configure extended DHCP server and DHCP relay agent and legacy DHCP server and DHCP relay agent in the same network.

  • You cannot configure extended DHCP server and DHCP relay agent and legacy DHCP server and DHCP relay agent on the same device. Because the newer extended DHCP server version has more features, we recommend that you configure the extended DHCP server if it is supported by the switch. A commit error is displayed if both legacy DHCPD and extended DHCP is configured simultaneously.

  • DHCP clients on a switch are always configured at the hierarchy level [edit interfaces interface-name family dhcp].

  • If you delete the DHCP server configuration, DHCP server bindings might still remain. To ensure that DHCP bindings are removed, issue the clear dhcp server binding command before you delete the DHCP server configuration.

Features Not Supported by Extended DHCP

  • Legacy DHCP supports the circuit ID and the remote ID fields for the relay agent option (option 82). Extended DHCP for the relay agent option supports only circuit ID. For more information on option 82, see Using DHCP Relay Agent Option 82 Information.

  • In Junos Release 12.1X46, autoinstallation is not compatible with JDHCPd: