Option Number and DescriptionIdentifiers a user-defined string used by the Relay Agent to forward DHCP clientpackets to a specific server.L2 DHCPSnoopingOption 82Specifies IP addresses for DHCP messages received from the client that are to bemonitored to build a DHCP snooping database.User Port Stacking Option 230Set the stacking option variable to provide DHCP server stack-port detail when theDHCP 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 DHCPservers. This message includes the parameters that the client requires and might include suggestedvalues for those parameters.2. Servers unicast or broadcast a DHCPOFFER message in response to the DHCPDISCOVER that offersto the client values for the requested parameters. Multiple servers might respond to a singleDHCPDISCOVER; the client might wait a period of time and then act on the most preferred offer.3. The client broadcasts a DHCPREQUEST message in response to the offer, requesting the offeredvalues.4. After receiving a DHCPREQUEST, the server binds the clients’ unique identifier (the hardware addressplus IP address) to the accepted configuration parameters and stores the data in a database called abinding table. The server then broadcasts a DHCPACK message, which signals to the client that itmay 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 aDHCPRELEASE message.There are additional messages that are used in case the DHCP negotiation deviates from the processpreviously described and shown in the illustration below.DHCPDECLINE A client sends this message to the server in response to a DHCPACK if theconfiguration parameters are unacceptable; for example, if the offered address isalready in use. In this case, the client starts the configuration process over bysending a DHCPDISCOVER.DHCPINFORM A client uses this message to request configuration parameters when it assigned anIP address manually rather 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 the requested address is already in use. In this case, the client startsthe configuration process over by sending a DHCPDISCOVER.288 Dynamic Host Configuration Protocol (DHCP)
