Concept Guide
Dynamic Host Conguration Protocol (DHCP)
DHCP is an application layer protocol that dynamically assigns IP addresses and other conguration parameters to network end-stations
(hosts) based on conguration policies determined by network administrators.
DHCP relieves network administrators of manually conguring hosts, which can be a tedious and error-prone process when hosts often
join, leave, and change locations on the network and it reclaims IP addresses that are no longer in use to prevent address exhaustion.
DHCP is based on a client-server model. A host discovers the DHCP server and requests an IP address, and the server either leases or
permanently assigns one. There are three types of devices that are involved in DHCP negotiation:
DHCP Server This is a network device oering conguration parameters to the client.
DHCP Client This is a network device requesting conguration parameters from the server.
Relay Agent This is an intermediary network device that passes DHCP messages between the client and server when the
server is not on the same subnet as the host.
Topics:
• DHCP Packet Format and Options
• Assign an IP Address using DHCP
• Implementation Information
• Congure the System to be a DHCP Server
• Congure the System to be a Relay Agent
• Congure the System to be a DHCP Client
• DHCP Relay When DHCP Server and Client are in Dierent VRFs
• Non-default VRF conguration for DHCPv6 helper address
• Conguring DHCP relay source interface
• Congure the System for User Port Stacking (Option 230)
• Congure Secure DHCP
• Source Address Validation
DHCP Packet Format and Options
DHCP uses the user datagram protocol (UDP) as its transport protocol.
The server listens on port 67 and transmits to port 68; the client listens on port 68 and transmits to port 67. The conguration parameters
are carried as options in the DHCP packet in Type, Length, Value (TLV) format; many options are specied in RFC 2132. To limit the number
of parameters that servers must provide, hosts specify the parameters that they require, and the server sends only those parameters.
Some common options are shown in the following illustration.
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