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 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.
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 starts the
configuration process over by sending a DHCPDISCOVER.
Figure 38. Client and Server Messaging
Implementation Information
The 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. If you configure IP source address validation on a
member port of a virtual local area network (VLAN) and then to apply an access list to the VLAN, Dell
Networking OS displays the first line in the following message. If you first apply an ACL to a VLAN and
then enable IP source address validation on one of its member ports, Dell Networking OS displays the
second line in the following message.
% Error: Vlan member has access-list configured.
% Error: Vlan has an access-list configured.
NOTE: If you enable DHCP Snooping globally and you have any configured L2 ports, any IP ACL,
MAC ACL, or DHCP source address validation ACL does not block DHCP packets.
Dynamic Host Configuration Protocol (DHCP) 346
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