DHCP Configuration Facts

DHCP Configuration Facts

Dynamic Host Configuration Protocol (DHCP) is a protocol used by hosts to obtain various parameters necessary for the clients to operate in a network. You can configure DHCP on a Cisco device through the command line interface (CLI) or the Security Device Manager (SDM). DHCP configuration parameters include the following:

Component	Description
Address pool	The address pool is the range of addresses which can be assigned to requesting hosts. The DHCP server only assigns addresses within the address pool. The DHCP server can also be configured to not assign specific addresses in the range, known as exclusions.
Lease	The lease is the length of time for which the assignment is valid. It contains the assigned IP address and other information for the client. Periodically and when the client reboots, it contacts the DHCP server to renew the lease on the IP address.
DHCP options	In addition to the IP address and subnet mask, the DHCP server can also deliver the following: Domain Name Server (DNS) server address(es) Default router (or default gateway) address WINS server addresses Additional TCP/IP configuration parameters
Binding	A binding is an association of a MAC address with a specific IP address. When you create a binding, the client with the specified MAC address is assigned the same IP address each time it requests an address. For example, if you have servers which should be accessible from outside the local network, the servers' IP addresses should remain the same. A binding is also known as DHCP reservation.
Interface	The interface that responds to DHCP requests is identified automatically according to the IP address assigned to the interface. When you configure the DHCP service on a Cisco device, it compares the subnet address specified in the address pool with the IP addresses assigned to the router interfaces. If the interface has been assigned an IP address in the address pool, that interface will listen for and respond to DHCP requests. To allow an interface to listen and respond to DHCP requests, assign it an IP address within the address pool. If the interface does not have an IP address, or if the IP address is not within the address pool, client DHCP requests will be ignored. You should exclude the interface IP address from the DHCP address pool.

A DHCP client uses the following process to obtain an IP address:

1. Lease Request. The client initializes a limited version of TCP/IP and broadcasts a DHCPDISCOVER packet requesting the location of a DHCP server.
2. Lease Offer. All DHCP servers with available IP addresses send DHCPOFFER packets to the client. These include the client's hardware address, the IP address the server is offering, the subnet mask, the duration of the IP lease, and the IP address of the DHCP server making the offer.
3. Lease Selection. The client selects the IP address from the first offer it receives and broadcasts a DHCPREQUEST packet requesting to lease the IP address in that offer.
4. IP Lease Acknowledgment. The DHCP server that made the offer responds and all other DHCP servers withdraw their offers. The IP addressing information is assigned to the client and the offering DHCP server sends a DHCPACK (acknowledgement) packet directly to the client. The client finishes initializing and binding the TCP/IP protocol.

The DHCP lease process uses frame-level broadcasts. For this reason, DHCP requests typically do not pass through routers to other subnets. To enable DHCP across subnets:

• Enable BootP (DHCP broadcast) requests through the router.
• Configure a computer for BootP forwarding to request IP information on behalf of other clients.

Credit: Testout 640-802 CCNA Notes