138Step Command Remarks10. (Optional.) Enable theswitchback to the masterDHCP server and set thedelay time.master-server switch-delaydelay-timeBy default, the DHCP relay agentdoes not switch back to the masterDHCP server.Configuring the DHCP relay agent securityfeaturesRustications and guidelinesIf you execute both the dhcp flood-protection enable and dhcp server check mac-addresscommands on an interface, the dhcp server check mac-address command takes priority.Enabling 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 features use the relay entries to check incoming packets and block packets that donot match any entry. In this way, illegal hosts are not able to access external networks through therelay agent. Examples of the security features are ARP address check, authorized ARP, and IPsource guard.The DHCP relay agent does not record IP-to-MAC bindings for DHCP clients running onsynchronous/asynchronous serial interfaces.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.Enabling periodic refresh of dynamic relay entriesA DHCP client unicasts a DHCP-RELEASE message to the DHCP server to release its IP address.The DHCP relay agent conveys the message to the DHCP server and does not remove theIP-to-MAC entry of the client.With this feature, the DHCP relay agent uses the following information to periodically send aDHCP-REQUEST message to the DHCP server:• The IP address of a relay entry.• The MAC address of the DHCP relay interface.The relay agent maintains the relay entries depending on what it receives from the DHCP server:• If the server returns a DHCP-ACK message or does not return any message within an interval,the DHCP relay agent removes the relay entry. In addition, upon receiving the DHCP-ACKmessage, 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.To enable periodic refresh of dynamic relay entries:
