the offers. Web Links DHCP Overview http://support.baynetworks.com/library/ tpubs/html/router/ soft1200/ 117362AA/A_29.HTM
Content 1.2 DHCP 1.2.4 DHCP operation The DHCP client configuration process uses the following steps:
  1. A client must have DHCP configured when starting the network membership process. The client sends a request to a server requesting an IP configuration. Sometimes the client may suggest the IP address it wants, such as when requesting an extension to a DHCP lease. The client locates a DHCP server by sending a broadcast called a DHCPDISCOVER.
  2. When the server receives the broadcast, it determines whether it can service the request from its own database. If it cannot, the server may forward the request on to another DHCP server. If it can, the DHCP server offers the client IP configuration information in the form of a unicast DHCPOFFER. The DHCPOFFER is a proposed configuration that may include IP address, DNS server address, and lease time.
  3. If the client finds the offer agreeable, it will send another broadcast, a DHCPREQUEST, specifically requesting those particular IP parameters. Why does the client broadcast the request instead of unicasting it to the server? A broadcast is used because the first message, the DHCPDISCOVER, may have reached more than one DHCP server. If more than one server makes an offer, the broadcasted DHCPREQUEST allows the other servers to know which offer was accepted. The offer accepted is usually the first offer received.
  4. The server that receives the DHCPREQUEST makes the configuration official by sending a unicast acknowledgment, the DHCPACK. It is possible, but highly unlikely, that the server will not send the DHCPACK. This may happen because the server may have leased that information to another client in the interim. Receipt of the DHCPACK message enables the client to begin using the assigned address immediately.
  5. If the client detects that the address is already in use on the local segment it will send a DHCPDECLINE message and the process starts again. If the client received a DHCPNACK from the server after sending the DHCPREQUEST, then it will restart the process again.
  6. If the client no longer needs the IP address, the client sends a DHCPRELEASE message to the server.
Depending on an organization's policies, it may be possible for an end user or an administrator to statically assign a host an IP address that belongs in the DHCP servers address pool. Just in case, the Cisco IOS DHCP server always checks to make sure that an address is not in use before the server offers it to a client. The server will issue an ICMP echo request, or will ping, to a pool address before sending the DHCPOFFER to a client. Although configurable, the default number of pings used to check for a potential IP address conflict is two. Web Links DHCP Operation http://www.cs.utexas.edu/users/ ypraveen/surveys/ DHCP/ node2.html
Content 1.2 DHCP 1.2.5 Configuring DHCP Like NAT, a DHCP server requires that the administrator define a pool of addresses. The ip dhcp pool command defines which addresses will be assigned to hosts.The first command, ip dhcp pool, creates a pool with the specified name and puts the router in a specialized DHCP configuration mode. In this mode, use the network statement to define the range of addresses to be leased. If specific addresses on the network are to be excluded, return to global configuration mode. The ip dhcp excluded-address command configures the router to exclude an individual address or range of addresses when assigning addresses to clients. The ip dhcp excluded-address command may be used to reserve addresses that are statically assigned to key hosts, for instance, the interface address on the router. Typically, a DHCP server will be configured to assign much more than an IP address. Other IP configuration values such as the default gateway can be set from the DHCP configuration mode. Using the default-router command sets the default gateway. The address of the DNS server, dns-server, and WINS server, netbios-name-server, can also be configured here. The IOS DHCP server can configure clients with virtually any TCP/IP information. A list of the key IOS DHCP server commands entered in the DHCP pool configuration mode are shown in Figure . The DHCP service is enabled by default on versions of Cisco IOS that support it. To disable the service, use the no service dhcp command. Use the service dhcp global configuration command to re-enable the DHCP server process. Web Links Configuring DHCP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_configuration_guide_ chapter09186a00800ca75c.html
Content 1.2 DHCP 1.2.6 Verifying DHCP operation To verify the operation of DHCP, the command show ip dhcp binding can be used. This displays a list of all bindings created by the DHCP service. To verify that messages are being received or sent by the router, use the command show ip dhcp server statistics. This will display count information regarding the number of DHCP messages that have been sent and received. Lab Activity Lab Exercise: Configuring DHCP In this lab, the student will configure a router for Dynamic Host Configuration Protocol (DHCP). Lab Activity e-Lab Activity: Configuring DHCP In this lab, the student will configure a router for DHCP, add the ability for workstations to remotely obtain DHCP addresses and dynamically assign addresses to the attached hosts. Web Links Configuring DHCP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/products_configuration_ guide_chapter09186a00800ca75c.html
Content 1.2 DHCP 1.2.7 Troubleshooting DHCP To troubleshoot the operation of the DHCP server, the command debug ip dhcp server events can be used. This command will show that the server periodically checks to see if any leases have expired. Also, it can be seen when addresses are returned and when they are allocated.
Content 1.2 DHCP 1.2.8 DHCP relay DHCP clients use IP broadcasts to find the DHCP server on the segment. What happens when the server and the client are not on the same segment and are separated by a router? Routers do not forward these broadcasts. DHCP is not the only critical service that uses broadcasts. Cisco routers and other devices may use broadcasts to locate TFTP servers. Some clients may need to broadcast to locate a TACACS server. A TACACS server is a security server. Typically, in a complex hierarchical network, clients reside on the same subnet as key servers. Such remote clients will broadcast to locate these servers. However, routers, by default, will not forward client broadcasts beyond their subnet. Because some clients are useless without services such as DHCP, one of two choices must be implemented. The administrator will need to place servers on all subnets or use the Cisco IOS helper address feature. Running services such as DHCP or DNS on several computers creates overhead and administrative difficulties making the first option inefficient. When possible, administrators should use the ip helper-address command to relay broadcast requests for these key UDP services. By using the helper address feature, a router can be configured to accept a broadcast request for a UDP service and then forward it as a unicast to a specific IP address. By default, the ip helper-address command forwards the following eight UDP services: In the particular case of DHCP, a client broadcasts a DHCPDISCOVER packet on its local segment. This packet is picked up by the gateway. If a helper-address is configured, the DHCP packet is forwarded to the specified address. Before forwarding the