Users Guide

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 following illustration.
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.
Figure 36. 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 congure IP source address validation on a member port of a virtual local area network (VLAN) and then attempt to apply an
access list to the VLAN, Dell Networking OS displays the rst line in the following message. If you rst apply an ACL to a VLAN
and then attempt 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 congured L2 ports, any IP ACL, MAC ACL, or DHCP
source address validation ACL does not block DHCP packets.
• Dell Networking OS provides 40K entries that can be divided between leased addresses and excluded addresses. By extension,
the maximum number of pools you can congure depends on the subnet mask that you give to each pool. For example, if all
pools were congured for a /24 mask, the total would be 40000/253 (approximately 158). If the subnet is increased, more pools
can be congured. The maximum subnet that can be congured for a single pool is /17. Dell Networking OS displays an error
message for congurations that exceed the allocated memory.
Dynamic Host Conguration Protocol (DHCP)
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