Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 370

Option L2 DHCP Snooping End Number and Description Identifiers a user-defined string used by the Relay Agent to forward DHCP client packets to a specific server. The MD field identifies if a DHCP discovery message was sent by BMP at bootup (MD=BMP) or the ip address dhcp command (MD=INT) in Interface configuration mode or Active Fabric Manager (AFM). Option 82 Specifies IP addresses for DHCP messages received from the client that are to be monitored to build a DHCP snooping database. Option 255 Signals the last option in the DHCP packet. Assign an IP Address using DHCP The 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 includes 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 for the requested parameters. Multiple servers might respond to a single DHCPDISCOVER; 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 offered values. 4. After receiving a DHCPREQUEST, the server binds the clients' unique identifier (the hardware address plus IP address) to the accepted configuration parameters and stores the data in a database called a binding table. The server then broadcasts a DHCPACK 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 DHCPRELEASE message. There are additional messages that are used in case the DHCP negotiation deviates from the process previously described and shown in the illustration below. DHCPDECLINE A client sends this message to the server in response to a DHCPACK if the configuration parameters are unacceptable; for example, if the offered address is already in use. In this case, the client starts the configuration process over by sending a DHCPDISCOVER. DHCPINFORM DHCPNAK A client uses this message to request configuration parameters when it assigned an IP address manually rather than with DHCP. The server responds by unicast. 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 starts the configuration process over by sending a DHCPDISCOVER. 370 Dynamic Host Configuration Protocol (DHCP)