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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 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 32. 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
attempt 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 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 configured 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 configure depends on the subnet mask that you give to each pool.
For example, if all pools were configured for a /24 mask, the total would be 40000/253 (approximately 158). If the subnet
is increased, more pools can be configured. The maximum subnet that can be configured for a single pool is /17. Dell
Networking OS displays an error message for configurations that exceed the allocated memory.
The S5000 supports 4K DHCP Snooping entries.
The S5000 supports Dynamic ARP Inspection on 16 VLANs per system. For more information, refer to Dynamic ARP
Inspection.
NOTE:
If the DHCP server is on the top of rack (ToR) and the VLTi (ICL) is down due to a failed link, when a VLT node
is rebooted in BMP (Bare Metal Provisioning) mode, it is not able to reach the DHCP server, resulting in BMP failure.
Configuration Tasks
Configure the System to be a DHCP Server
Configure the System to be a Relay Agent
Configure Secure DHCP
Dynamic Host Configuration Protocol (DHCP)
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