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DHCP Overview
The Dynamic Host Configuration Protocol (DHCP) is based on the Bootstrap Protocol (BOOTP), which
provides the framework for passing configuration information to hosts on a TCP/IP network. DHCP adds
the capability to automatically allocate reusable network addresses and configuration options to Internet
hosts. DHCP consists of two components: a protocol for delivering host-specific configuration parameters
from a DHCP server to a host and a mechanism for allocating network addresses to hosts. DHCP is built on
a client/server model, where designated DHCP server hosts allocate network addresses and deliver
configuration parameters to dynamically configured hosts.
This module describes the concepts needed to understand Cisco IOS XE DHCP.
• Finding Feature Information, page 1
• Information About DHCP, page 2
• Additional References for DHCP Overview, page 6
• Technical Assistance, page 7
• Glossary, page 7
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and
feature information, see Bug Search Tool and the release notes for your platform and software release. To
find information about the features documented in this module, and to see a list of the releases in which each
feature is supported, see the feature information table.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
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DHCP Overview
Information About DHCP
Information About DHCP
DHCP Overview
Cisco routers running Cisco IOS XE software include Dynamic Host Control Protocol (DHCP) server and
relay agent software. The Cisco IOS XE DHCP server is a full DHCP server implementation that assigns and
manages IP addresses from specified address pools within the router to DHCP clients. These address pools
can also be configured to supply additional information to the requesting client such as the IP address of the
DNS server, the default router, and other configuration parameters. If the Cisco IOS XE DHCP server cannot
satisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP
servers defined by the network administrator.
DHCP supports three mechanisms for IP address allocation:
• Automatic allocation—DHCP assigns a permanent IP address to a client.
• Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time, which is called
a lease (or until the client explicitly relinquishes the address). DHCP also supports on-demand address
pools (ODAPs), which is a feature in which pools of IP addresses can be dynamically increased or
reduced in size depending on the address utilization level. ODAPs support address assignment for
customers using private addresses.
• Manual allocation—The network administrator assigns an IP address to a client and DHCP is used
simply to convey the assigned address to the client.
The format of DHCP messages is based on the format of BOOTP messages, which ensures support for BOOTP
relay agent functionality and interoperability between BOOTP clients and DHCP servers. BOOTP relay agents
eliminate the need for deploying a DHCP server on each physical network segment. BOOTP is explained in
RFC 951, Bootstrap Protocol (BOOTP), and RFC 1542, Clarifications and Extensions for the Bootstrap
Protocol.
The main advantage of DHCP compared to BOOTP is that DHCP does not require that the DHCP server be
configured with all MAC addresses of all clients. DHCP defines a process by which the DHCP server knows
the IP subnet in which the DHCP client resides, and it can assign an IP address from a pool of valid IP addresses
in that subnet. Most of the other information that DHCP might supply, such as the default router IP address,
is the same for all hosts in the subnet so DHCP servers can usually configure information per subnet rather
than per host. This functionality reduces network administration tasks compared to BOOTP.
Benefits of Using DHCP
The DHCP implementation offers the following benefits:
• Reduced Internet access costs
Using automatic IP address assignment at each remote site substantially reduces Internet access costs.
Static IP addresses are considerably more expensive to purchase than are automatically allocated IP
addresses.
• Reduced server configuration tasks and costs
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DHCP Overview
DHCP Server Relay Agent and Client Operation
Because DHCP is easy to configure, it minimizes operational overhead and costs associated with device
configuration tasks and eases deployment by nontechnical users.
• Centralized management
Because the DHCP server maintains configurations for several subnets, an administrator only needs to
update a single, central server when configuration parameters change.
DHCP Server Relay Agent and Client Operation
Dynamic Host Control Protocol (DHCP) provides a framework for passing configuration information
dynamically to hosts on a TCP/IP network. A DHCP client is a host that uses DHCP to obtain configuration
parameters such as an IP address.
A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents are
used to forward requests and replies between clients and servers when they are not on the same physical
subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams
are switched between networks somewhat transparently. By contrast, relay agents receive DHCP messages
and then generate a new DHCP message to send on another interface.
The figure below shows the basic steps that occur when a DHCP client requests an IP address from a DHCP
server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a DHCP server. A relay
agent forwards the packets between the DHCP client and server. A DHCP server offers configuration parameters
(such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client in a
DHCPOFFER unicast message.
Figure 1: DHCP Request for an IP Address from a DHCP Server
A DHCP client may receive offers from multiple DHCP servers and can accept any one of the offers; however,
the client usually accepts the first offer it receives. Additionally, the offer from the DHCP server is not a
guarantee that the IP address will be allocated to the client; however, the server usually reserves the address
until the client has had a chance to formally request the address.
The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST
broadcast message. The DHCP server confirms that the IP address has been allocated to the client by returning
a DHCPACK unicast message to the client.
The formal request for the offered IP address (the DHCPREQUEST message) that is sent by the client is
broadcast so that all other DHCP servers that received the DHCPDISCOVER broadcast message from the
client can reclaim the IP addresses that they offered to the client.
If the configuration parameters sent to the client in the DHCPOFFER unicast message by the DHCP server
are invalid (a misconfiguration error exists), the client returns a DHCPDECLINE broadcast message to the
DHCP server.
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DHCP Overview
DHCP Database
The DHCP server will send to the client a DHCPNAK denial broadcast message, which means the offered
configuration parameters have not been assigned, if an error has occurred during the negotiation of the
parameters or the client has been slow in responding to the DHCPOFFER message (the DHCP server assigned
the parameters to another client) of the DHCP server.
DHCP Database
DHCP address pools are stored in non-volatile RAM (NVRAM). There is no limit on the number of address
pools. An address binding is the mapping between the client’s IP and hardware addresses. The client’s IP
address can be configured by the administrator (manual address allocation) or assigned from a pool by the
DHCP server.
Manual bindings are stored in NVRAM. Manual bindings are just special address pools configured by a
network administrator. There is no limit on the number of manual bindings.
Automatic bindings are IP addresses that have been automatically mapped to the MAC addresses of hosts
that are found in the DHCP database. Automatic bindings are stored on a remote host called the database
agent. A DHCP database agent is any host--for example, an FTP, TFTP, or RCP server--that stores the DHCP
bindings database.The bindings are saved as text records for easy maintenance.
You can configure multiple DHCP database agents and you can configure the interval between database
updates and transfers for each agent.
DHCP Attribute Inheritance
The DHCP server database is organized as a tree. The root of the tree is the address pool for natural networks,
branches are subnetwork address pools, and leaves are manual bindings to clients. Subnetworks inherit network
parameters and clients inherit subnetwork parameters. Therefore, common parameters (for example, the
domain name) should be configured at the highest (network or subnetwork) level of the tree.
Inherited parameters can be overridden. For example, if a parameter is defined in both the natural network
and a subnetwork, the definition of the subnetwork is used.
Address leases are not inherited. If a lease is not specified for an IP address, by default, the DHCP server
assigns a one-day lease for the address.
DHCP Options and Suboptions
Configuration parameters and other control information are carried in tagged data items that are stored in the
options field of the DHCP message. Options provide a method of appending additional information. Vendors
that want to provide additional information to their client not designed into the protocol can use options.
The Cisco IOS XE DHCP implementation also allows most DHCP server options to be customized. For
example, the TFTP server, which stores the Cisco IOS XE image, can be customized with option 150 to
support intelligent IP phones.
Virtual Private Networks (VPNs) allow the possibility that two pools in separate networks can have the same
address space, with private network addresses, served by the same DHCP server. Cisco IOS XE software
supports VPN-related options and suboptions such as the relay agent information option and VPN identification
suboption. A relay agent can recognize these VPN-related options and suboptions and forward the
client-originated DHCP packets to a DHCP server. The DHCP server can use this information to assign IP
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DHCP Overview
DHCP Options and Suboptions
addresses and other parameters, distinguished by a VPN identifier, to help select the VPN to which the client
belongs.
For more information on DHCP options and suboptions, see the “DHCP Options Reference” appendix in the
Network Registrar User’s Guide , Release 6.3.
During lease negotiation, the DHCP server sends the options shown in the table below to the client.
Table 1: Default DHCP Server Options
DHCP Option Name
DHCP Option Code
Description
Subnet mask option
1
Specifies the client’s subnet mask per RFC
950.
Router option
3
Specifies a list of IP addresses for routers
on the client’s subnet, usually listed in order
of preference.
Domain name server option
6
Specifies a list of DNS name servers
available to the client, usually listed in
order of preference.
Hostname option
12
Specifies the name of the client. The name
may or may not be qualified with the local
domain name.
Domain name option
15
Specifies the domain name that the client
should use when resolving hostnames via
the Domain Name System.
NetBIOS over TCP/IP name server option 44
Specifies a list of RFC 1001/1002 NetBIOS
name servers listed in order or preference.
NetBIOS over TCP/IP node type option
46
Enables NetBIOS over TCP/IP clients that
are configurable to be configured as
described in RFC 1001/1002.
IP address lease time option
51
Allows the client to request a lease for the
IP address.
DHCP message type option
53
Conveys the type of the DHCP message.
Server identifier option
54
Identifies the IP address of the selected
DHCP server.
Renewal (T1) time option
58
Specifies the time interval from address
assignment until the client transitions to
the renewing state.
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DHCP Overview
DHCP Server On-Demand Address Pool Management Overview
DHCP Option Name
DHCP Option Code
Description
Rebinding (T2) time option
59
Specifies the time interval from address
assignment until the client transitions to
the rebinding state.
DHCP Server On-Demand Address Pool Management Overview
The Cisco IOS DHCP server on-demand address pool (ODAP) manager is used to centralize the management
of large pools of addresses and simplify the configuration of large networks. ODAP provides a central
management point for the allocation and assignment of IP addresses. When a router is configured as an ODAP
manager, pools of IP addresses are dynamically increased or reduced in size depending on the address utilization
level.
ODAPs support address assignment using DHCP for customers using private addresses. Each ODAP is
configured and associated with a particular Multiprotocol Label Switching (MPLS) VPN. Cisco IOS software
also provides ODAP support for non-MPLS VPN address pools by adding pool name support to the peer
default ip address dhcp-pool pool namecommand.
DHCP server subnet allocation is a way of offering entire subnets (ranges of addresses) to relay agents so that
remote access devices can provision IP addresses to DHCP clients. This functionality can occur along with
or instead of managing individual client addresses. Subnet allocation can improve IP address provisioning,
aggregation, characterization, and distribution by relying on the DHCP infrastructure to dynamically manage
subnets.
This capability allows the DHCP server to be configured with a pool of subnets for lease to ODAP clients.
Subnet pools can be configured for global ODAP clients or MPLS VPN ODAP clients on a per-client basis.
The DHCP subnet allocation server creates bindings for the subnet leases and stores these leases in the DHCP
database.
Additional References for DHCP Overview
Related Documents
Related Topic
Document Title
Cisco IOS commands
Cisco IOS Master Command List, All Releases
DHCP commands
Cisco IOS IP Addressing Services Command
Reference
IPv6 commands
Cisco IOS IPv6 Command Reference
IPv6 addressing and connectivity
IPv6 Configuration Guide
Cisco IOS IPv6 features
Cisco IOS IPv6 Feature Mapping
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DHCP Overview
Technical Assistance
Standards and RFCs
Standard/RFC
Title
RFC 951
Bootstrap Protocol (BOOTP)
RFC 1542
Clarifications and Extensions for the Bootstrap
Protocol
RFCs for IPv6
IPv6 RFCs
Technical Assistance
Description
Link
The Cisco Support and Documentation website
http://www.cisco.com/cisco/web/support/index.html
provides online resources to download documentation,
software, and tools. Use these resources to install and
configure the software and to troubleshoot and resolve
technical issues with Cisco products and technologies.
Access to most tools on the Cisco Support and
Documentation website requires a Cisco.com user ID
and password.
Technical Assistance
Description
Link
The Cisco Support website provides extensive online http://www.cisco.com/techsupport
resources, including documentation and tools for
troubleshooting and resolving technical issues with
Cisco products and technologies.
To receive security and technical information about
your products, you can subscribe to various services,
such as the Product Alert Tool (accessed from Field
Notices), the Cisco Technical Services Newsletter,
and Really Simple Syndication (RSS) Feeds.
Access to most tools on the Cisco Support website
requires a Cisco.com user ID and password.
Glossary
address binding—A mapping between the client’s IP and hardware (MAC) addresses. The client’s IP address
may be configured by the administrator (manual address allocation) or assigned from a pool by the DHCP
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DHCP Overview
Glossary
server (automatic address allocation). The binding also contains a lease expiration date. The default for the
lease expiration date is one day.
address conflict—A duplication of use of the same IP address by two hosts. During address assignment,
DHCP checks for conflicts using ping and gratuitous (ARP). If a conflict is detected, the address is removed
from the pool. The address will not be assigned until the administrator resolves the conflict.
address pool—The range of IP addresses assigned by the DHCP server. Address pools are indexed by subnet
number.
automatic address allocation --An address assignment method where a network administrator obtains an IP
address for a client for a finite period of time or until the client explicitly relinquishes the address. Automatic
allocation is particularly useful for assigning an address to a client that will be connected to the network only
temporarily or for sharing a limited pool of IP addresses among a group of clients that do not need permanent
IP addresses. Automatic allocation may also be a good choice for assigning an IP address to a new client being
permanently connected to a network where IP addresses are sufficiently scarce that it is important to reclaim
them when old clients are retired.
BOOTP—Bootstrap Protocol. A protocol that provides a method for a booting computer to find out its IP
address and the location of the boot file with the rest of its parameters.
client—Any host requesting configuration parameters.
database—A collection of address pools and bindings.
database agent—Any host storing the DHCP bindings database, for example, a Trivial File Transfer Protocol
(TFTP) server.
DHCP—Dynamic Host Configuration Protocol. A protocol that provides a mechanism for allocating IP
addresses dynamically so that addresses can be reused when hosts no longer need them.
DNS—Domain Name System. A system used in the Internet for translating names of network nodes into
addresses.
manual address allocation—An address assignment method that allocates an administratively assigned IP
address to a host. Manual allocation allows DHCP to be used to eliminate the error-prone process of manually
configuring hosts with IP addresses.
PWLAN—Public Wireless Local Area Network. A type of wireless LAN, often referred to as a hotspot, that
anyone having a properly configured computer device can access.
relay agent—A device that forwards DHCP and BOOTP messages between a server and a client on different
subnets.
server—Any host providing configuration parameters.
SSG—Service Selection Gateway. The feature set that provides on-demand service enforcement within the
Cisco network.
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