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 Relay Agent Information Reforwarding Policy
Viewing DHCP Relay Configuration and Statistics
Configuring the DHCP Relay
Dynamic Host Configuration Protocol (DHCP) servers provide configuration parameters to DHCP clients. DHCP is a protocol that 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 client 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 address for the client. When the DHCP server responds, the DHCP relay agent forwards the reply back it to the DHCP client.
Note
The ACE does not support DHCP relay for DHCP broadcast packets received on shared VLANs.
This chapter includes the following major sections:
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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 5-1 illustrates the basic steps that occur when a DHCP client requests an IP address from a DHCP server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a DHCP server. A relay agent forwards the packets between the DHCP client and server. A DHCP server offers configuration parameters (such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.
Figure 5-1 DHCP Request for an IP Address from a DHCP Server
DHCP Relay Configuration Quick Start
Table 5-1 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 5-1.
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 negotiating between the DHCP clients and the server. By default, the DHCP relay agent is disabled. You must configure a DHCP server in conjunction with enabling the DHCP relay.
The DHCP relay agent can be configured at both the context and VLAN interface level of the ACE. Note the following configuration considerations:
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This section includes the following topics:
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Enabling the DHCP Relay
To accept DHCP requests from clients on the associated context or VLAN interface and enable the DHCP relay agent, use the ip dhcp relay enable command. The DHCP relay starts forwarding packets to the DHCP server address specified in the ip dhcp relay server command for the associated VLAN interface or context.
The syntax of this command is:
ip dhcp relay enable
For example, to enable the DHCP relay to all interfaces associated with a context, enter:
host1/Admin(config)# ip dhcp relay enableFor example, to enable the DHCP relay at the VLAN interface level, enter:
host1/Admin(config)# interface vlan 50host1/Admin(config-if)# ip dhcp relay enableTo disable the DHCP relay for all interfaces associated with a context, enter:
host1/Admin(config)# no ip dhcp relay enableTo disable the DHCP relay on the VLAN interface, enter:
host1/Admin(config-if)# no ip dhcp relay enableSpecifying the DHCP Server IP Address
To set the IP address of a DHCP server to which the DHCP relay agent forwards client requests, use the ip dhcp relay server command.
The syntax of this command is:
ip dhcp relay server ip_address
The ip_address argument specifies the IP address of the DHCP server. Enter the address in dotted-decimal IP notation (for example, 192.168.20.1).
For example, to set the IP address of a DHCP relay server to all interfaces associated with a context, enter:
host1/Admin(config)# ip dhcp relay enablehost1/Admin(config)# ip dhcp relay server 192.168.20.1For example, to set the IP 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 IP address of a DHCP server, enter:
host1/Admin(config-if)# no ip dhcp relay server 192.168.20.1Configuring a Relay Agent Information Reforwarding Policy
To configure the DHCP relay to identify what is to be performed if a forwarded message already contains relay information, use the ip dhcp relay information policy command in configuration mode. By default, the reforwarding policy is to drop the DHCP relay packet.
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The syntax of this command is:
ip dhcp relay information policy {keep | replace}
The keywords are:
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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
To view configuration information and statistics collected for the DHCP relay, use the show ip dhcp relay command. There are three show commands for DHCP relay:
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The output of this command increments until you enter the clear ip dhcp relay statistics command.
For example, to display the configured status of the relay agent information reforwarding policy, enter:
host/Admin# show ip dhcp relay information policyDHCP Relay reforwarding policy configured = REPLACETo clear all of the DHCP relay statistics information, use the clear ip dhcp relay statistics command. For example, enter:
host1/Admin# clear ip dhcp relay statisticsTable 5-2 describes the fields in the show ip dhcp relay conf command output.
Table 5-3 describes the fields in the show ip dhcp relay statistics command output.
