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Implementing IP Addressing ServicesAccessing the WAN – Chapter 7

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Every device that connects to a network needs an IP address. Network administrators assign static IP addresses to routers, servers, and other network devices whose locations (physical and logical) are not likely to change. Administrators enter static IP addresses manually when they configure devices to join the network. Static addresses also enable administrators to manage those devices remotely. However, computers in an organization often change locations, physically and logically.

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Nội dung Text: Implementing IP Addressing ServicesAccessing the WAN – Chapter 7

  1. Implementing IP Addressing Services Accessing the WAN – Chapter 7 1 ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  2. Cisco Thai Nguyen Networking Academy Objectives Configure DHCP in an enterprise branch network Configure NAT on a Cisco router Configure new generation RIP (RIPng) to use IPv6 2 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  3. Cisco Thai Nguyen Networking Academy Introducing DHCP Every device that connects to a network needs an IP address. Network administrators assign static IP addresses to routers, servers, and other network devices whose locations (physical and logical) are not likely to change. Administrators enter static IP addresses manually when they configure devices to join the network. Static addresses also enable administrators to manage those devices remotely. However, computers in an organization often change locations, physically and logically. Administrators are unable to keep up with having to assign new IP addresses every time an employee moves to a different office or cubicle. Desktop clients do not require a static address. Instead, a workstation can use any address within a range of addresses. This range is typically within an IP subnet DHCP assigns IP addresses and other important network configuration information dynamically. Because desktop clients typically make up the bulk of network nodes, DHCP is an extremely useful and timesaving tool for network administrators. RFC 2131 describes DHCP. 3 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  4. Cisco Thai Nguyen Networking Academy Introducing DHCP Administrators typically prefer a network server to offer DHCP services, because these solutions are scalable and relatively easy to manage. However, in a small branch or SOHO location, a Cisco router can be configured to provide DHCP services without the need for an expensive dedicated server. A Cisco IOS feature set called Easy IP offers an optional, full-featured DHCP server. 4 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  5. Cisco Thai Nguyen Networking Academy DHCP Operation Providing IP addresses to clients is the most fundamental task performed by a DHCP server. DHCP includes three different address allocation mechanisms to provide flexibility when assigning IP addresses: Manual Allocation: The administrator assigns a pre-allocated IP address to the client and DHCP only communicates the IP address to the device. Automatic Allocation: DHCP automatically assigns a static IP address permanently to a device, selecting it from a pool of available addresses. There is no lease and the address is permanently assigned to a device. Dynamic Allocation: DHCP automatically dynamically assigns, or leases, an IP address from a pool of addresses for a limited period of time chosen by the server, or until the client tells the DHCP server that it no longer needs the address. 5 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  6. Cisco Thai Nguyen Networking Academy BOOTP vs. DHCP The Bootstrap Protocol (BOOTP), defined in RFC 951, is the predecessor of DHCP and shares some operational characteristics. BOOTP is a way to download address and boot configurations for diskless workstations. A diskless workstation does not have a hard drive or an operating system. For example, many automated cash register systems at your local supermarket are examples of diskless workstations Both DHCP and BOOTP are client/server based and use UDP ports 67 and 68. Those ports are still known as BOOTP ports. 6 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  7. Cisco Thai Nguyen Networking Academy BOOTP vs. DHCP There are three primary differences between DHCP and BOOTP: The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. DHCP allows for recovery and reallocation of network addresses through a leasing mechanism. Specifically, DHCP defines mechanisms through which clients can be assigned an IP address for a finite lease period. This lease period allows for reassignment of the IP address to another client later, or for the client to get another assignment if the client moves to another subnet. Clients may also renew leases and keep the same IP address. BOOTP does not use leases. BOOTP provides a limited amount of information to a host. DHCP provides additional IP configuration parameters, such as WINS and domain name 7 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  8. Cisco Thai Nguyen Networking Academy DHCP Server Configuration Cisco routers running Cisco IOS software provide full support for a router to act as a DHCP server. The Cisco IOS DHCP server assigns and manages IP addresses from specified address pools within the router to DHCP clients. 8 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  9. Cisco Thai Nguyen Networking Academy 9 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  10. Cisco Thai Nguyen Networking Academy 10 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  11. Cisco Thai Nguyen Networking Academy 11 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  12. Cisco Thai Nguyen Networking Academy Verify DHCP Server Configuration To verify the operation of DHCP, use the show ip dhcp binding command. This command displays a list of all IP address to MAC address bindings that have been provided by the DHCP service. To verify that messages are being received or sent by the router, use the show ip dhcp server statistics command. This command displays count information regarding the number of DHCP messages that have been sent and received. 12 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  13. Cisco Thai Nguyen Networking Academy DHCP Client Configuration Typically, small broadband routers for home use, such as Linksys routers, can be configured to connect to an ISP using a DSL or cable modem. In most cases, small home routers are set to acquire an IP address automatically from their ISPs. For example, the figure shows the default WAN setup page for a Linksys WRVS4400N router. Notice that the Internet connection type is set to Automatic Configuration - DHCP. This means that when the router is connected to a cable modem, for example, it is a DHCP client and requests an IP address from the ISP. 13 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  14. Cisco Thai Nguyen Networking Academy DHCP Client Configuration Cisco routers in SOHO and branch sites have to be configured in a similar manner. The method used depends on the ISP. However, in its simplest configuration, the Ethernet interface is used to connect to a cable modem. To configure an Ethernet interface as a DHCP client, the ip address dhcp command must be configured. In the figure, assume that an ISP has been configured to provide select customers with IP addresses from the 209.165.201.0 / 27 range. The ouput confirms the assigned address. 14 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  15. Cisco Thai Nguyen Networking Academy DHCP Relay In a complex hierarchical network, enterprise servers are usually contained in a server farm. These servers may provide DHCP, DNS, TFTP, and FTP services for the clients. The problem is that the network clients typically are not on the same subnet as those servers. Therefore, the clients must locate the servers to receive services and often these services are located using broadcast messages. In the figure, PC1 is attempting to acquire an IP address from the DHCP server located at 192.168.11.5. In this scenario router R1 is not configured as a DHCP server. 15 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  16. Cisco Thai Nguyen Networking Academy DHCP Relay A simpler solution is to configure the Cisco IOS helper address feature on intervening routers and switches. This solution enables routers to forward DHCP broadcasts to the DHCP servers. When a router forwards address assignment/parameter requests, it is acting as a DHCP relay agent. For example, PC1 would broadcast a request to locate a DHCP server. If router R1 were configured as a DHCP relay agent, it would intercept this request and forward it to the DHCP server located on subnet 192.168.11.0. To configure router R1 as a DHCP relay agent, you need to configure the nearest interface to the client with the ip helper-address interface configuration command. This command relays broadcast requests for key services to a configured address. Configure the IP helper address on the interface receiving the broadcast. Router R1 is now configured as a DHCP relay agent. It accepts broadcast requests for the DHCP service and then forwards them as a unicast to the IP address 192.168.11.5. 16 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  17. Cisco Thai Nguyen Networking Academy DHCP Configuration with SDM Cisco routers can also be configured as a DHCP server using SDM. In this example, router R1 will be configured as the DHCP server on the Fa0/0 and Fa0/1interfaces. The DHCP server function is enabled under Additional Tasks in the Configure tab. From the list of tasks, click on the DHCP folder and then select DHCP Pools to add a new pool. Click Add to create the new DHCP pool. 17 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  18. Cisco Thai Nguyen Networking Academy DHCP Configuration with SDM The Add DHCP Pool window contains the options you need to configure the DHCP IP address pool. The IP addresses that the DHCP server assigns are drawn from a common pool. To configure the pool, specify the starting and ending IP addresses of the range. 18 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  19. Cisco Thai Nguyen Networking Academy DHCP Configuration with SDM This screen provides you with a summary of the pools configured on your router. In this example, there have been two pools configured, one for each of the Fast Ethernet interfaces on the R1 router. 19 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
  20. Cisco Thai Nguyen Networking Academy Troubleshooting DHCP DHCP problems can arise for a multitude of reasons, such as software defects in operating systems, NIC drivers, or DHCP/BOOTP relay agents, but the most common are configuration issues. Because of the number of potentially problematic areas, a systematic approach to troubleshooting is required. show ip dhcp conflict 20 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
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