Feedback
Table Of Contents
DHCP Server and Client Overview
DHCP Relay Configuration Quick Start
Configuring the DHCP Relay Agent
Specifying the DHCP Server IP Address
Configuring a Forwarding Interface for DHCPv6 Relay
Configuring a Relay Agent Information Reforwarding Policy
Viewing DHCP Relay Configuration and Statistics
Configuring the DHCP Relay
Note
The information in this chapter applies to both the ACE module and the ACE appliance unless otherwise noted. The features described in this chapter apply to IPv4 and IPv6 unless otherwise noted.
This chapter describes how Dynamic Host Configuration Protocol (DHCP) servers provide configuration parameters to DHCP clients. DHCP supplies network settings, including the host IP address, the default gateway, and a DNS server. When DHCP clients and associated servers do not reside on the same IP network or subnet, a DHCP relay agent can transfer DHCP messages between them. The DHCP relay agent operates as the interface between DHCP clients and the server. It listens for client requests and adds vital configuration data, such as the client's link information, which is needed by the server to allocate the address for the client. When the DHCP server responds, the DHCP relay agent forwards the reply back to the DHCP client.
Note
This chapter contains the following major sections:
•
•
•
•
DHCP Server and Client Overview
DHCP provides a framework for passing configuration information dynamically to hosts on a TCP/IP network. A DHCP client is an Internet host using DHCP to obtain configuration parameters such as an IP address.
A DHCP relay agent is any host that forwards DHCP packets between clients and servers. 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. By contrast, relay agents receive DHCP messages and then generate a new DHCP message to send on another interface.
Figure 8-1 shows the basic steps that occur when a DHCPv6 client requests an IP address from a DHCPv6 server. The client, Host A, sends a DHCPSOLICIT multicast message to locate a DHCPv6 server. A relay agent forwards the packets between the DHCPv6 client and server. A DHCPv6 server offers configuration parameters (for example, an IPv6 address, a MAC address, a domain name, NTP server, and a lease for the IPv6 address) to the client in a DHCPRELAY-FORW unicast message.
Figure 8-1 DHCPv6 Request for an IPv6 Address from a DHCP Server
Figure 8-2 shows the basic steps that occur when a DHCPv4 client requests an IPv4 address from a DHCPv4 server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a DHCPv4 server. A relay agent forwards the packets between the DHCPv4 client and server. A DHCPv4 server offers configuration parameters (for example, an IPv4 address, a MAC address, a domain name, NTP server, and a lease for the IPv4 address) to the client in a DHCPOFFER unicast message.
Figure 8-2 DHCPv4 Request for an IPv4 Address from a DHCP Server
DHCP Relay Configuration Quick Start
Table 8-1 provides a quick overview of the steps required to configure the DHCP relay function on the ACE. Each step includes the CLI command required to complete the task. For a complete description of each feature and all the options associated with the CLI command, see the sections following Table 8-1.
Configuring the DHCP Relay Agent
This section describes how to configure the DHCP relay agent on the ACE. When you configure the ACE as a DHCP relay agent, it is responsible for forwarding the requests and responses that are negotiated between the DHCP clients and the server. By default, the DHCP relay agent is disabled. You must configure a DHCP server when you enable the DHCP relay agent.
You can configure the DHCP relay agent at both the context and VLAN interface levels of the ACE as follows:
•
•
This section contains the following topics:
•
•
Enabling the DHCP Relay
You can accept DHCP requests from clients on the associated context or VLAN interface and enable the DHCP relay agent by using the ipv6 dhcp relay enable command (for IPv6) or the ip dhcp relay enable command (for IPv4). The DHCP relay starts forwarding packets to the DHCP server address specified in the ipv6 dhcp relay server command or the ip dhcp relay server command for the associated context or VLAN interface.
IPv6 Syntax
The syntax of this command is as follows:
ipv6 dhcp relay enable
IPv6 Examples
To enable the DHCP relay agent globally for all VLAN interfaces associated with a context, enter the following command:
host1/Admin(config)# ipv6 dhcp relay enableTo enable the DHCP relay agent at the VLAN interface level, enter the following command:
host1/Admin(config)# ipv6 dhcp relay enablehost1/Admin(config)# interface vlan 100host1/Admin(config-if)# ipv6 dhcp relay enableTo disable the DHCP relay agent globally for VLAN interfaces in a context where DHCP relay is not explicitly configured, enter the following command:
host1/Admin(config)# no ipv6 dhcp relay enableTo disable the DHCP relay agent on a VLAN interface, enter the following commands:
host1/Admin(config)# interface vlan 100host1/Admin(config-if)# no ipv6 dhcp relay enableIPv4 Syntax
The syntax of this command is as follows:
ip dhcp relay enable
IPv4 Examples
To enable the DHCP relay agent for all interfaces associated with a context, enter the following command:
host1/Admin(config)# ip dhcp relay enableFor example, to enable the DHCP relay agent at the VLAN interface level, enter the following command:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# ip dhcp relay enableTo disable the DHCP relay agent for interfaces in a context where DHCP relay is not configured explicitly, enter the following command:
host1/Admin(config)# no ip dhcp relay enableTo disable the DHCP relay agent on a VLAN interface, enter the following commands:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# no ip dhcp relay enableSpecifying the DHCP Server IP Address
You can set the IP address of a DHCP server to which the DHCP relay agent forwards client requests by using either the ipv6 dhcp relay server command or the ip dhcp relay server command.
IPv6 Syntax
The syntax of this command is as follows:
ipv6 dhcp relay server ipv6_address [fwd-interface vlan vlan_id]
The keywords and arguments are as follows:
•
•
IPv6 Examples
To set the IPv6 address of a DHCPv6 relay server globally for all interfaces associated with a context, enter:
host1/Admin(config)# ipv6 dhcp relay enablehost1/Admin(config)# ipv6 dhcp relay server 2001:DB8:1::123To set the IPv6 address of a DHCP relay server at the VLAN interface level, enter:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# ipv6 dhcp relay enablehost1/Admin(config-if)# ipv6 dhcp relay server 2001:DB8:1::/64To set the IPv6 address of a DHCPv6 server that is reachable on its link-local address, enter the following commands:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# ipv6 dhcp relay enablehost1/Admin(config-if)# ipv6 dhcp relay server fe80::250:56ff:fe90:2c fwd-interface vlan 100
Note
To remove the IP address of a DHCP server from a VLAN interface, enter:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# no ipv6 dhcp relay server 2001:DB8:1::/64IPv4 Syntax
The syntax of this command is as follows:
ip dhcp relay server ip_address
The ip_address argument specifies the IPv4 address of the DHCP server.
IPv4 Examples
To set the IPv4 address of a DHCP relay server on all interfaces associated with a context, enter:
host1/Admin(config)# ip dhcp relay enablehost1/Admin(config)# ip dhcp relay server 192.168.20.1To set the IPv4 address of a DHCP relay server at the VLAN interface level, enter:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# ip dhcp relay enablehost1/Admin(config-if)# ip dhcp relay server 192.168.20.1To remove the IPv4 address of a DHCP server from an interface, enter:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# no ip dhcp relay server 192.168.20.1Configuring a Forwarding Interface for DHCPv6 Relay
For IPv6, you can configure a forwarding VLAN interface that the ACE uses to forward all client requests on the specified VLAN interface to the All_DHCP_Relay_Agents_and_Servers address of FF02::1:2. To configure the DHCP forwarding VLAN interface, use the ipv6 dhcp relay fwd-interface command in either configuration mode or interface configuration mode. The syntax of this command is as follows:
ipv6 dhcp relay fwd-interface vlan vlan_id
The vlan_id argument specifies the VLAN interface number that the ACE uses to forward DHCP requests. Enter the VLAN interface number of an existing VLAN interface as an integer from 2 to 4094.
For example, to configure VLAN200 as the DHCP forwarding VLAN interface, enter the following command:
host1/Admin(config)# ipv6 dhcp relay fwd-interface vlan 200To remove the forwarding VLAN interface from the configuration, enter the following command:
host1/Admin(config)# no ipv6 dhcp relay fwd-interface vlan 200Configuring a Relay Agent Information Reforwarding Policy
For IPv4, you can configure the DHCP relay agent to identify the action to perform if a forwarded message already contains relay information by using the ip dhcp relay information policy command in configuration mode. By default, the reforwarding policy is to drop the DHCP relay packet.
Note
The syntax of this command is as follows:
ip dhcp relay information policy {keep | replace}
The keywords are as follows:
•
•
For example, to set the relay agent information reforwarding policy to replace existing information for all interfaces associated with a context, enter:
host1/Admin(config)# ip dhcp relay information policy replaceTo restore the default relay information policy to drop the DHCP relay packet, enter:
host1/Admin(config)# no ip dhcp relay information policy replaceViewing DHCP Relay Configuration and Statistics
You can view configuration information and statistics collected for the DHCP relay agent by using the show ipv6 dhcp relay command or the show ip dhcp relay command for IPv4.
IPv6 DHCP Relay Show Commands
There are two show commands for IPv6 DHCP relay as follows:
•
•
For example, to display the configured status of the DHCP relay agent and the DHCP server configuration, enter the following command:
host/Admin# show ipv6 dhcp relayTo clear all the DHCP relay statistics, use the clear ipv6 dhcp relay statistics command. For example, enter the following command:
host1/Admin# clear ipv6 dhcp relay statisticsTable 8-4 describes the fields in the show ipv6 dhcp relay command output.
Table 8-5 describes the fields in the show ipv6 dhcp relay statistics command output.
IPv4 DHCP Relay Show Commands
There are three show commands for IPv4 DHCP relay as follows:
•
•
•
For example, to display the configured status of the DHCP relay information reforwarding policy, enter:
host/Admin# show ip dhcp relay information policyDHCP Relay reforwarding policy configured = REPLACETo clear all the DHCP relay statistics information, use the clear ip dhcp relay statistics command. For example, enter:
host1/Admin# clear ip dhcp relay statisticsTable 8-4 describes the fields in the show ip dhcp relay conf command output.
Table 8-5 describes the fields in the show ip dhcp relay statistics command output.
