Configuring the DHCP Relay Agent 593Configuring the DHCPRelay Agent to Send aDHCP-Release RequestSometimes, you need to release a client’s IP address manually on the DHCP relayagent. With this task completed, the DHCP relay agent can actively send aDHCP-RELEASE request that contains the client’s IP address to be released. Uponreceiving the DHCP-RELEASE request, the DHCP server then releases the IP addressfor the client.To configure the DHCP relay agent to send a DHCP-RELEASE request, use thefollowing commands:Configuring the DHCPRelay Agent SecurityFunctionsCreating static bindings and enable invalid IP address checkThe DHCP relay agent can dynamically record clients’ IP-to-MAC bindings togenerate a dynamic binding after clients got IP addresses. It also supports staticbinding, which means you can manually configure IP-to-MAC bindings on theDHCP relay agent, so that users can access external network using fixed IPaddresses.For avoidance of invalid IP address configuration, you can configure the DHCPrelay agent to check whether a requesting client’s IP and MAC addresses match abinding on it (both dynamic and static bindings). If not, the client cannot accessoutside networks via the DHCP relay agent.To create a static binding and enable invalid IP address check, use the followingcommands:n ■ The dhcp relay address-check command can be executed only on Layer 3Ethernet interfaces (including sub-interfaces) and VLAN interfaces.■ The dhcp relay address-check enable command is independent of othercommands of the DHCP relay agent. That is, the invalid address check takeseffect when this command is executed, regardless of whether other commandsare used.■ You are recommended to configure IP address check on the interface enabledwith the DHCP relay agent; otherwise, the valid DHCP clients may not becapable of accessing networks.To do... Use the command... RemarksEnter system view system-view -Configure the DHCP relay agent to send aDHCP-RELEASE requestdhcp relay release ipclient-ipRequiredTo do... Use the command... RemarksEnter system view system-view -Create a static binding dhcp relay security staticip-address mac-address[ interface interface-typeinterface-number ]OptionalNo static binding is created bydefaultEnter interface view interface interface-typeinterface-number-Enable invalid IP addresscheckdhcp relay address-check{ disable | enable }RequiredDisabled by default
