4000 Router Series Layer 3 - IP Services Configuration Guide

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To specify a DHCP server address on a relay agent:

Ste

Command

Remarks

1. Enter system view.

system-view N/A

2. Enter interface view.

interface interface-type

interface-number

N/A

3. Specify a DHCP server

address on the relay agent.

dhcp relay server-address

ip-address

By default, no DHCP server

address is specified on the relay

agent.

Configuring the DHCP relay agent security

functions

Enabling the DHCP relay agent to record relay entries

Perform this task to enable the DHCP relay agent to automatically record clients' IP-to-MAC bindings

(relay entries) after they obtain IP addresses through DHCP.

Some security functions, such as ARP address check, authorized ARP, and IP source guard, use the

recorded relay entries to check incoming packets and block packets that do not match any entry. In this

way, illegal hosts are not able to access external networks through the relay agent.

To enable the DHCP relay agent to record relay entries:

Ste

Command

Remarks

1. Enter system view.

system-view N/A

2. Enable the relay agent to

record relay entries.

dhcp relay client-information record

By default, the relay agent

does not record relay entries.

NOTE:

The DHCP relay agent does not record IP-to-MAC bindings for DHCP clients running on

synchronous/asynchronous serial interfaces.

Enabling periodic refresh of dynamic relay entries

A DHCP client unicasts a DHCP-RELEASE message to the DHCP server to release its IP address. The

DHCP relay agent simply conveys the message to the DHCP server and does not remove the IP-to-MAC

entry of the client.

With this feature, the DHCP relay agent uses the IP address of a relay entry and the MAC address of the

DHCP relay interface to periodically send a DHCP-REQUEST message to the DHCP server.

• If the server returns a DHCP-ACK message or does not return any message within a specific interval,

the DHCP relay agent removes the relay entry. In addition, upon receiving the DHCP-ACK message,

the relay agent sends a DHCP-RELEASE message to release the IP address.

• If the server returns a DHCP-NAK message, the relay agent keeps the relay entry.