Chapter 1: LAN Technologies - DHCP PDF

Chapter 1: LAN Technologies 1.0 LAN TECHNOLOGIES 1.2.6 DHCP DHCPv4 Concepts DHCPv4 Server and Client Dynamic Host Configuration Protocol v4 (DHCPv4) assigns IPv4 addresses and other network configuration information dynamically. Because desktop clients typically make up the bulk of...

Chapter 1: LAN Technologies 1.0 LAN TECHNOLOGIES 1.2.6 DHCP DHCPv4 Concepts DHCPv4 Server and Client Dynamic Host Configuration Protocol v4 (DHCPv4) assigns IPv4 addresses and other network configuration information dynamically. Because desktop clients typically make up the bulk of network nodes, DHCPv4 is an extremely useful and timesaving tool for network administrators. A dedicated DHCPv4 server is scalable and relatively easy to manage. However, in a small branch or SOHO location, a Cisco router can be configured to provide DHCPv4 services without the need for a dedicated server. Cisco IOS software supports an optional, full-featured DHCPv4 server. DHCPv4 Concepts DHCPv4 Server and Client The DHCPv4 server dynamically assigns, or leases, an IPv4 address from a pool of addresses for a limited period of time chosen by the server, or until the client no longer needs the address. Clients lease the information from the server for an administratively defined period. Administrators configure DHCPv4 servers to set the leases to time out at different intervals. The lease is typically anywhere from 24 hours to a week or more. When the lease expires, the client must ask for another address, although the client is typically reassigned the same address. DHCPv4 Concepts DHCPv4 Operation DHCPv4 works in a client/server mode. When a client communicates with a DHCPv4 server, the server assigns or leases an IPv4 address to that client. The client connects to the network with that leased IPv4 address until the lease expires. The client must contact the DHCP server periodically to extend the lease. This lease mechanism ensures that clients that move or power off do not keep addresses that they no longer need. When a lease expires, the DHCP server returns the address to the pool where it can be reallocated as necessary. DHCPv4 Concepts Steps to Obtain a Lease When the client boots (or otherwise wants to join a network), it begins a four-step process to obtain a lease: 1. DHCP Discover (DHCPDISCOVER) 2. DHCP Offer (DHCPOFFER) 3. DHCP Request (DHCPREQUEST) 4. DHCP Acknowledgment (DHCPACK) DHCPv4 Concepts Steps to Renew a Lease Prior to lease expiration, the client begins a two- step process to renew the lease with the DHCPv4 server, as shown in the figure: 1. DHCP Request (DHCPREQUEST) Before the lease expires, the client sends a DHCPREQUEST message directly to the DHCPv4 server that originally offered the IPv4 address. If a DHCPACK is not received within a specified amount of time, the client broadcasts another DHCPREQUEST so that one of the other DHCPv4 servers can extend the lease. Note: These messages (primarily the DHCPOFFER and DHCPACK) can be sent as unicast or broadcast according 2. DHCP Acknowledgment (DHCPACK) to IETF RFC 2131. On receiving the DHCPREQUEST message, the server verifies the lease information by returning a DHCPACK. Configure a Cisco IOS DHCPv4 Server Cisco IOS DHCPv4 Server Now you have a basic understanding of how DHCPv4 works and how it can make your job a bit easier. A Cisco router running Cisco IOS software can be configured to act as a DHCPv4 server. The Cisco IOS DHCPv4 server assigns and manages IPv4 addresses from specified address pools within the router to DHCPv4 clients. Configure a Cisco IOS DHCPv4 Server Steps to Configure a Cisco IOS DHCPv4 Server Use the following steps to configure a Cisco IOS DHCPv4 server: Step 1. Exclude IPv4 addresses. A single address or a range of addresses can be excluded by specifying the low-address and high-address of the range. Excluded addresses should be those addresses that are assigned to routers, servers, printers, and other devices that have been, or will be, manually configured. You can also enter the command multiple times. The command is ip dhcp excluded-address low-address [high-address] Step 2. Define a DHCPv4 pool name. The ip dhcp pool pool-name command creates a pool with the specified name and puts the router in DHCPv4 configuration mode, which is identified by the prompt Router(dhcp-config)#. Configure a Cisco IOS DHCPv4 Server Steps to Configure a Cisco IOS DHCPv4 Server (Cont.) Step 3. Configure the DHCPv4 pool. The address pool and default gateway router must be configured. Use the network statement to define the range of available addresses. Use the default-router command to define the default gateway router. These commands and other optional commands are shown in the table. Task IOS Command Define the address pool. network network-number [mask | / prefix-length] Define the default router or gateway. default-router address [ address2….address8] Define a DNS server. dns-server address [ address2…address8] Define the domain name. domain-name domain Define the duration of the DHCP lease. lease {days [hours [ minutes]] | infinite} Define the NetBIOS WINS server. netbios-name-server address [ address2…address8] Configure a Cisco IOS DHCPv4 Server Configuration Example Configure a Cisco IOS DHCPv4 Server DHCPv4 Verification Use the commands in the table to verify that the Cisco IOS DHCPv4 server is operational. Command Description show running-config | section dhcp Displays the DHCPv4 commands configured on the router. Displays a list of all IPv4 address to MAC address bindings provided by the DHCPv4 show ip dhcp binding service. Displays count information regarding the number of DHCPv4 messages that have show ip dhcp server statistics been sent and received Configure a Cisco IOS DHCPv4 Server Verify DHCPv4 is Operational Verify the DHCPv4 Configuration: As shown in the example, the show running-config | section dhcp command output displays the DHCPv4 commands configured on R1. The | section parameter displays only the commands associated with DHCPv4 configuration. Configure a Cisco IOS DHCPv4 Server Verify DHCPv4 is Operational (Cont.) Verify DHCPv4 Bindings: As shown in the example, the operation of DHCPv4 can be verified using the show ip dhcp binding command. This command displays a list of all IPv4 address to MAC address bindings that have been provided by the DHCPv4 service. Configure a Cisco IOS DHCPv4 Server Verify DHCPv4 is Operational (Cont.) Verify DHCPv4 Statistics: The output of the show ip dhcp server statistics is used to verify that messages are being received or sent by the router. This command displays count information regarding the number of DHCPv4 messages that have been sent and received. Configure a Cisco IOS DHCPv4 Server Verify DHCPv4 is Operational (Cont.) Verify DHCPv4 Client Received IPv4 Addressing: The ipconfig /all command, when issued on PC1, displays the TCP/IP parameters, as shown in the example. Because PC1 was connected to the network segment 192.168.10.0/24, it automatically received a DNS suffix, IPv4 address, subnet mask, default gateway, and DNS server address from that pool. No DHCP-specific router interface configuration is required. If a PC is connected to a network segment that has a DHCPv4 pool available, the PC can obtain an IPv4 address from the appropriate pool automatically. Configure a Cisco IOS DHCPv4 Server Disable the Cisco IOS DHCPv4 Server The DHCPv4 service is enabled by default. To disable the service, use the no service dhcp global configuration mode command. Use the service dhcp global configuration mode command to re-enable the DHCPv4 server process, as shown in the example. Enabling the service has no effect if the parameters are not configured. Note: Clearing the DHCP bindings or stopping and restarting the DHCP service may result in duplicate IP addresses being temporarily assigned on the network. Configure a Cisco IOS DHCPv4 Server DHCPv4 Relay In a complex hierarchical network, enterprise servers are usually located centrally. These servers may provide DHCP, DNS, TFTP, and FTP services for the network. Network clients are not typically on the same subnet as those servers. In order to locate the servers and receive services, clients often use broadcast messages. In the figure, PC1 is attempting to acquire an IPv4 address from a DHCPv4 server using a broadcast message. In this scenario, R1 is not configured as a DHCPv4 server and does not forward the broadcast. Because the DHCPv4 server is located on a different network, PC1 cannot receive an IP address using DHCP. R1 must be configured to relay DHCPv4 messages to the DHCPv4 server. Configure a Cisco IOS DHCPv4 Server DHCPv4 Relay (Cont.) Configure R1 with the ip helper-address address interface configuration command. This will cause R1 to relay DHCPv4 broadcasts to the DHCPv4 server. As shown in the example, the interface on R1 receiving the broadcast from PC1 is configured to relay DHCPv4 address to the DHCPv4 server at 192.168.11.6. When R1 has been configured as a DHCPv4 relay agent, it accepts broadcast requests for the DHCPv4 service and then forwards those requests as a unicast to the IPv4 address 192.168.11.6. The network administrator can use the show ip interface command to verify the configuration. Configure a Cisco IOS DHCPv4 Server Packet Tracer – Configure DHCPv4 In this Packet Tracer Activity, you will complete the following objectives: Part 1: Configure a Router as a DHCP Server Part 2: Configure DHCP Relay Part 3: Configure a Router as a DHCP Client Part 4: Verify DHCP and Connectivity Configure a DHCPv4 Client Cisco Router as a DHCPv4 Client There are scenarios where you might have access to a DHCP server through your ISP. In these instances, you can configure a Cisco IOS router as a DHCPv4 client. Sometimes, Cisco routers in a small office or home office (SOHO) and branch sites have to be configured as DHCPv4 clients in a similar manner to client computers. The method used depends on the ISP. However, in its simplest configuration, the Ethernet interface is used to connect to a cable or DSL modem. To configure an Ethernet interface as a DHCP client, use the ip address dhcp interface configuration mode command. In the figure, assume that an ISP has been configured to provide select customers with IP addresses from the 209.165.201.0/27 network range after the G0/0/1 interface is configured with the ip address dhcp command. Configure a DHCPv4 Client Configuration Example To configure an Ethernet interface as a DHCP client, use the ip address dhcp interface configuration mode command, as shown in the example. This configuration assumes that the ISP has been configured to provide select customers with IPv4 addressing information. The show ip interface g0/1 command confirms that the interface is up and that the address was allocated by a DHCPv4 server. Configure a DHCPv4 Client Home Router as a DHCPv4 Client Home routers are typically already set to receive IPv4 addressing information automatically from the ISP. This is so that customers can easily set up the router and connect to the internet. For example, the figure shows the default WAN setup page for a Packet Tracer wireless router. Notice that the internet connection type is set to Automatic Configuration - DHCP. This selection is used when the router is connected to a DSL or cable modem and acts as a DHCPv4 client, requesting an IPv4 address from the ISP. Various manufacturers of home routers will have a similar setup.