Option Number and DescriptionUser Port Stacking Option 230Set the stacking option variable to provide DHCP server stack-port detail when the DHCP offer is set.End Option 255Signals the last option in the DHCP packet.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 message includesthe 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 for therequested parameters. Multiple servers might respond to a single DHCPDISCOVER; the client might wait a period of time and thenact 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 a DHCPACKmessage, 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 DHCPRELEASE message.There are additional messages that are used in case the DHCP negotiation deviates from the process previously described and shown inthe 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 the configurationprocess over by sending a DHCPDISCOVER.DHCPINFORM A client uses this message to request configuration parameters when it assigned an IP address manually ratherthan 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 the requestedaddress is already in use. In this case, the client starts the configuration process over by sending aDHCPDISCOVER.Figure 36. Client and Server Messaging288 Dynamic Host Configuration Protocol (DHCP)
