58To specify a DHCP server address on a relay agent:Step Command Remarks1. Enter system view. system-view N/A2. Enter interface view. interface interface-typeinterface-number N/A3. Specify a DHCP serveraddress on the relay agent.dhcp relay server-addressip-addressBy default, no DHCP serveraddress is specified on the relayagent.Configuring the DHCP relay agent securityfunctionsEnabling the DHCP relay agent to record relay entriesPerform 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 therecorded relay entries to check incoming packets and block packets that do not match any entry. In thisway, illegal hosts are not able to access external networks through the relay agent.To enable the DHCP relay agent to record relay entries:Step Command Remarks1. Enter system view. system-view N/A2. Enable the relay agent torecord relay entries. dhcp relay client-information record By default, the relay agentdoes not record relay entries.NOTE:The DHCP relay agent does not record IP-to-MAC bindings for DHCP clients running onsynchronous/asynchronous serial interfaces.Enabling periodic refresh of dynamic relay entriesA DHCP client unicasts a DHCP-RELEASE message to the DHCP server to release its IP address. TheDHCP relay agent simply conveys the message to the DHCP server and does not remove the IP-to-MACentry of the client.With this feature, the DHCP relay agent uses the IP address of a relay entry and the MAC address of theDHCP 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.
