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Search is currently unavailable due to technical issues. We are working to resolve the problem as quickly as possible. This chapter describes how to configure Dynamic Host Configuration Protocol DHCP. For a complete description of the DHCP commands listed in this chapter, refer to the "DHCP Commands" chapter of the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services publication. To locate documentation of other commands that appear in this chapter, use the command reference master index, or search online. As explained in RFCDynamic Host Configuration ProtocolDHCP provides configuration parameters 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. By default, Cisco routers running Cisco IOS software include DHCP server and relay agent software. The format of DHCP messages cisco based on the format of Bootstrap Protocol 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. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter in this book. The Cisco IOS DHCP Server feature is a full DHCP Server implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. Figure shows the basic steps that occur when a DHCP client requests an IP address from a DHCP 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. Note 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. Note Options 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 existsthe client returns a DHCPDECLINE broadcast message to the DHCP Server. 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 cisco has been slow in responding to the DHCPOFFER message the DHCP Server assigned the parameters to another client of the DHCP Server. DHCP defines a process by which the DHCP Server knows the IP subnet in which the DHCP client options, and it can assign an IP address from a pool of valid IP addresses in that subnet. If the interface has secondary IP addresses, the subnets associated with the secondary IP addresses are examined for possible allocation only after the subnet associated with the primary IP address on the interface is exhausted. Static IP addresses are considerably more expensive to purchase than are automatically allocated IP addresses. 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. Relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface. The Cisco IOS DHCP relay agent supports the use of unnumbered interfaces. The DHCP relay agent automatically adds a static host route specifying the unnumbered interface as the outbound interface. 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. Note 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. First configure a database agent or disable conflict logging, then specify IP addresses that the DHCP Server should not assign excluded addresses and should assign a pool of available IP addresses to requesting clients. The tasks in the first three sections are required. The tasks in the remaining sections are optional. A DHCP database agent is any host—for example, an FTP, TFTP, or rcp server—that stores the DHCP bindings database. You can configure multiple DHCP database agents and you can configure the interval between database updates and transfers for each agent. If you choose not to configure a DHCP database agent, disable the recording of Options address conflicts on the DHCP Server. To disable Options address conflict logging, use the following command in global configuration mode: Router config no ip dhcp conflict logging The DHCP Server assumes that all IP addresses in a DHCP address pool subnet are available for assigning to DHCP clients. You must specify the IP address that the DHCP Server should not assign to clients. You can configure a DHCP address pool with cisco name that is a symbolic string such as "engineering" or cisco integer such as Configuring a DHCP address pool also places you in DHCP pool configuration mode—identified by the dhcp-config prompt—from which you can configure pool parameters for example, the IP subnet number and default router list. To configure a DHCP address pool, complete the required tasks in the following sections. To configure the DHCP address pool name and enter DHCP pool configuration mode, use the following command in global configuration mode: Router config ip dhcp pool name Creates a name for options DHCP Server address pool and places you in DHCP pool configuration mode identified by the dhcp-config prompt. The prefix is an alternative way of specifying the network mask of the client. The prefix length must be preceded by a forward slash Note You can not configure manual bindings within the same pool that is configured with the network command. To configure manual bindings, see the "Configuring Manual Bindings" section. The domain name for options DHCP client places the client in the general grouping of networks that make up the domain. To configure a domain name string for the client, use the following command in DHCP pool configuration mode: Router dhcp-config domain-name domain DHCP options query DNS IP servers when they need to correlate host names to IP addresses. One IP address is required; however, you can specify up to eight IP addresses in one command line. Windows Internet Naming Service WINS is a name resolution service that Microsoft DHCP clients use to correlate host names to IP addresses within a general grouping of networks. One address is required; however, you can specify up to eight addresses in one command line. The NetBIOS node type for Microsoft DHCP clients can be one of four settings: broadcast, peer-to-peer, mixed, or hybrid. To configure the NetBIOS node type for a Microsoft DHCP, use the following command in DHCP pool configuration mode: Router dhcp-config netbios-node-type type After a DHCP client has booted, the client begins sending packets to its default router. The IP address of the default router should be on the same subnet as the client. One IP address is required; however, you can specify up to eight addresses in one command line. By default, each IP address assigned by a DHCP Server comes with a one-day lease, which is the amount of time that the address is valid. An address binding is a mapping between the IP address and MAC address of a client. The IP address of a client can be assigned manually by an administrator or assigned automatically from a pool by a DHCP server. Manual bindings are IP addresses that have been manually mapped to the MAC cisco of hosts that are found in the DHCP database. Manual bindings are stored in NVRAM on the DHCP server. Manual bindings are just special address pools. There is no limit on the number of manual bindings, but you can only configure one manual binding per host pool. 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 a database agent. The bindings are saved as text records for easy maintenance. All DHCP clients send a client identifier DHCP option in the DHCP packet. To configure manual bindings, you must enter the client-identifier DHCP pool configuration command with the appropriate hexadecimal values identifying the DHCP client. To configure a manual binding, first create a host pool, then specify the IP address of the client and client identifier or hardware address. To configure manual bindings, use the following commands beginning in global configuration mode: Router config ip dhcp pool name Creates a name for the a DHCP Server address pool and places you in DHCP pool configuration mode—identified by the dhcp-config prompt. The prefix length must be preceded by a forward slash Router dhcp-config client-identifier unique-identifier Specifies the unique identifier for DHCP clients. This command is used for DHCP requests. The unique identification of the client is specified in dotted hexadecimal notation, for example, 01bwhere 01 represents the Ethernet media type. Router dhcp-config hardware-address hardware-address type Optional Specifies a hardware address for the client. This command is used for BOOTP requests. Strings and values are acceptable. The client name should not include the domain name. You can determine the client identifier by using the debug ip dhcp server packet command. In the following example, the client is identified by the value 0b cisco Router debug ip dhcp server packet DHCPD:DHCPDISCOVER received from client 0b a029 through relay DHCPD:assigned IP address to client 0b a Configuring a DHCP Server Boot File The boot file is used to store the boot image options the client. The boot image is generally the operating system the client uses to load. To specify a boot file for the DHCP client, use the following command in DHCP pool configuration mode: Router dhcp-config bootfile filename By default, the DHCP Server pings a pool address twice before assigning a particular address to a requesting client. If the ping is unanswered, the DHCP Server assumes with a high probability that the address is not in use and assigns the address to the requesting client. To change the number of ping packets the DHCP Server should send to the pool address before assigning the address, use the following command in global configuration mode: Router config ip dhcp ping packets number Specifies the number of ping packets the DHCP Server sends to a pool address before assigning the address to a requesting client. The default is two packets. Setting the count argument to a value of 0 turns off DHCP Server ping operation completely. By default, the DHCP Server waits milliseconds before timing out a ping packet. To change the amount of time the server waits, use the following command in global configuration mode: Router config ip dhcp ping timeout milliseconds Specifies the amount of time the DHCP Server must wait before timing out a ping packet. The default is milliseconds. Previously, network administrators needed to manually configure the Cisco IOS DHCP server on each device enabled with this feature. The Cisco IOS DHCP server was enhanced to allow configuration information to be updated automatically. Network administrators can configure one or more centralized DHCP servers to update specific DHCP options within the DHCP pools. The remote servers can request or "import" these option parameters from the centralized servers. See the section "DHCP Server Options Import and Autoconfiguration Example" later in this chapter for a configuration example. To configure the central router to update specific DHCP options within the DHCP pools, use the following commands beginning in global configuration mode: Router config ip dhcp pool name Creates a name for the a DHCP Server address pool and places you in DHCP pool configuration mode—identified by the dhcp-config prompt. To configure the remote router to import DHCP options into the DHCP server database, use the following commands beginning in global configuration mode: Router config ip dhcp pool name Creates a name for the a DHCP Server address pool and places you in DHCP pool configuration mode—identified by the dhcp-config prompt. By default, the DHCP smart-relay feature is disabled. To enable the smart-relay functionality, use the following command in global configuration mode: Router config ip dhcp smart-relay Allows the DHCP relay agent to switch the gateway address giaddr field of a DHCP packet to secondary addresses when there is no DHCPOFFER message from a DHCP Server. You can also use this command to display the IP addresses that have already been assigned. Router show ip dhcp server statistics Displays count information about server statistics and messages sent and received. Router show ip dhcp import Displays the option parameters that were imported into the DHCP Server database. Imported option parameters are not part of the router configuration and are not saved in NVRAM. The server should wait 2 minutes seconds before writing database changes. The following example shows a remote and central server configured to support DHCP options import and autoconfiguration. The central server is configured to automatically update DHCP options, such as DNS and WINs addresses, within the DHCP pools. In response to a DHCP request from a local client cisco CPE equipment, the remote server can request or "import" these option parameters from the centralized server. See Figure for a diagram of the network topology. Specifies network number and mask for DHCP clients network! Specifes the domain name for the client options central! Specifies DNS server that will respond to DHCP clients when they need to correlate host! Specifies the NETBIOS WINS server netbios-name-server! To locate documentation of other commands that appear in this chapter, use the command reference master index, or search online As explained in RFCDynamic Host Configuration ProtocolDHCP provides configuration parameters to Internet hosts. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter in this book DHCP Server Overview The Cisco IOS DHCP Server feature is a full DHCP Server implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. 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 Note 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. Relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface The Cisco IOS DHCP relay agent supports the use of unnumbered interfaces. The DHCP relay agent automatically adds a static host route options the unnumbered interface as the outbound interface DHCP Configuration Task List The DHCP Server database is organized as a tree. Therefore, common parameters, for example the domain name, should be configured at the highest network or subnetwork level of the tree Note Inherited parameters can be overridden. To disable DHCP address conflict logging, use the following command in global configuration mode Command Purpose Router config no ip dhcp conflict logging Disables DHCP address conflict logging Excluding IP Addresses The DHCP Cisco assumes that all IP addresses in a DHCP address pool subnet are available for assigning to DHCP clients. To configure manual bindings, see the "Configuring Manual Bindings" cisco Configuring the Domain Name for the Client The domain name for a DHCP client places the client in the general cisco of networks that make up the domain. To configure a domain name string for the client, use the following command in DHCP pool configuration mode Command Purpose Router dhcp-config domain-name domain Specifies the domain name for the client Configuring the IP Domain Name System Servers for the Client DHCP clients query DNS IP servers when they need to correlate host names to IP addresses. One IP address is required; however, you can specify up to eight IP addresses in one command line Configuring the NetBIOS Windows Internet Naming Service Servers for the Client Windows Internet Naming Service WINS is a name resolution service that Cisco DHCP clients use to correlate host names to IP addresses within a general grouping of networks. One address is required; however, you can specify up to eight addresses in one command line Configuring the NetBIOS Node Type for the Client The NetBIOS node type for Microsoft DHCP clients can be one of four settings: broadcast, peer-to-peer, mixed, or hybrid. To configure the NetBIOS node type for a Microsoft DHCP, use the following command in DHCP pool configuration mode Command Purpose Router dhcp-config netbios-node-type type Specifies the NetBIOS node type for a Microsoft DHCP client Configuring the Default Router for the Client After a DHCP client has booted, the client begins sending packets to its default router. One IP address is required; however, cisco can specify up to eight addresses in one command line Configuring the Address Lease Time By default, each IP address assigned by a DHCP Server comes with a one-day lease, which is the amount of time that the address is valid. The IP address of a client can be assigned manually by an administrator or assigned automatically from a pool by a DHCP server Manual bindings are IP addresses that have been manually mapped to the MAC addresses of hosts that are found in the DHCP database. There is no limit on the number of manual bindings, but you can only configure one manual binding per host pool Automatic bindings are IP addresses that have been options mapped to the MAC addresses of hosts that are found in the DHCP database. Cisco bindings are saved as text records for easy maintenance All DHCP clients send a client identifier DHCP option in the DHCP packet. The prefix length must be preceded by a forward slash Step Router dhcp-config client-identifier unique-identifier Specifies the unique identifier for DHCP clients. To specify a boot file for the DHCP client, use the following command in DHCP pool configuration mode Command Purpose Router dhcp-config bootfile filename Specifies the name of the file that is used as a boot image Configuring the Number of Ping Packets By default, the DHCP Server pings a pool address twice before assigning a particular address to a requesting client. To change the number of ping packets the DHCP Server should send to the pool address before assigning the address, use the following command in global configuration mode Command Purpose Router config ip dhcp ping packets number Specifies the number of ping packets the DHCP Server sends to a pool address before assigning the address to a requesting client. Setting the count argument to a value of 0 turns off DHCP Server ping operation completely Configuring the Timeout Value for Ping Packets By default, the DHCP Options waits milliseconds before timing out a ping packet. To change the amount of time the server waits, use the following command in global configuration mode Command Purpose Router config ip dhcp ping timeout milliseconds Specifies the amount of time the DHCP Server must wait before timing out a ping packet. Imports DHCP options parameters into DHCP server database import all network! Setting the count argument to a value of 0 turns off DHCP Server ping operation completely Router config ip dhcp ping timeout milliseconds Specifies the amount of time the DHCP Server must wait before timing out a ping packet.
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