Concept Guide

Option Number and Description
Rebinding Time Option 59
Species the amount of time after the IP address is granted that the client attempts to renew its lease with any
server, if the original server does not respond.
Vendor Class
Identifer
Option 60
Identies a user-dened string used by the Relay Agent to forward DHCP client packets to a specic server.
L2 DHCP Snooping Option 82
Species IP addresses for DHCP messages received from the client that are to be monitored to build a DHCP
snooping database.
User Port Stacking Option 230
Set the stacking option variable to provide DHCP server stack-port detail when the DHCP oer is set.
End 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 oers 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 oer.
3 The client broadcasts a DHCPREQUEST message in response to the oer, requesting the oered values.
4 After receiving a DHCPREQUEST, the server binds the clients’ unique identier (the hardware address plus IP address) to the
accepted conguration 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 conguration parameters are
unacceptable; for example, if the oered address is already in use. In this case, the client starts the conguration
process over by sending a DHCPDISCOVER.
DHCPINFORM A client uses this message to request conguration parameters when it assigned an IP 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 fulll a DHCPREQUEST; for example, if the requested
address is already in use. In this case, the client starts the conguration process over by sending a
DHCPDISCOVER.
Dynamic Host Conguration Protocol (DHCP) 263