Assign an IP Address using DHCPThe following section describes DHCP and the client in a network.When a client joins a network:1. The client initially broadcasts a DHCPDISCOVER message on the subnet to discover available DHCP servers. This messageincludes the parameters that the client requires and might include suggested values for those parameters.2. Servers unicast or broadcast a DHCPOFFER message in response to the DHCPDISCOVER that offers to the client values forthe requested parameters. Multiple servers might respond to a single DHCPDISCOVER; the client might wait a period of timeand then act on the most preferred offer.3. The client broadcasts a DHCPREQUEST message in response to the offer, requesting the offered values.4. After receiving a DHCPREQUEST, the server binds the clients’ unique identifier (the hardware address plus IP address) to theaccepted configuration parameters and stores the data in a database called a binding table. The server then broadcasts aDHCPACK message, which signals to the client that it may begin using the assigned parameters.5. When the client leaves the network, or the lease time expires, returns its IP address to the server in a DHCPRELEASEmessage.There are additional messages that are used in case the DHCP negotiation deviates from the process previously described andshown in the illustration below.DHCPDECLINE A client sends this message to the server in response to a DHCPACK if the configuration parameters areunacceptable; for example, if the offered address is already in use. In this case, the client starts theconfiguration process over by sending a DHCPDISCOVER.DHCPINFORM A client uses this message to request configuration parameters when it assigned an IP address manuallyrather than with DHCP. The server responds by unicast.DHCPNAK A server sends this message to the client if it is not able to fulfill a DHCPREQUEST; for example, if therequested address is already in use. In this case, the client starts the configuration process over by sending aDHCPDISCOVER.Figure 38. Client and Server MessagingImplementation InformationThe following describes DHCP implementation.• Dell Networking implements DHCP based on RFC 2131 and RFC 3046.• IP source address validation is a sub-feature of DHCP Snooping; the Dell Networking OS uses access control lists (ACLs)internally to implement this feature and as such, you cannot apply ACLs to an interface which has IP source address validation. IfDynamic Host Configuration Protocol (DHCP) 283
