Bài giảng Network+ Certification - Chapter 9, TCP/IP Routing

Chapter 9, TCP/IP Routing |1| Chapter Overview A. Routing Principles B. Building Routing Tables Chapter 9, Lesson 1 Routing Principles |2| 1. Understanding Routing A. A router is a system connected to two or more networks that forwards packets from one network to another. B. Routers operate at the network layer of the Open Systems Interconnection (OSI) reference model. C. Routers can connect networks running different data-link layer protocols and different network media. D. On large internetworks, each network often has more than one router connected to it. 1. Redundant routers provide multiple routes to the same destination. E. One of a router’s primary functions is to select the most efficient route to a destination. |3| 1. In most cases, the most efficient route is the one with the fewest hops. F. Routers share information about the networks they are attached to with other routers in the immediate vicinity. 1. Routers develop a composite picture of the internetwork by using this method. 2. On a large internetwork such as the Internet, no single router possesses an image of the entire network. |4| 2. Router Products A. Routers can be stand-alone hardware products or regular computers. B. Routing capabilities in operating systems 1. Microsoft Windows 2000, Microsoft Windows NT, and Novell NetWare can route IP traffic between two networks. a. To route IP, you must install two network interface adapters, connect them to different networks, and configure the computer to act as a router. b. A computer with two or more network interfaces is called a multihomed system. 2. Microsoft Windows 95, Microsoft Windows 98, and Microsoft Windows Me cannot route IP traffic between two network interface adapters on their own. a. You can use these operating systems as dial-in servers to access a network from a remote location using the NetBIOS Extended User Interface (NetBEUI) or Internetwork Packet Exchange (IPX) protocol. 2 Outline, Chapter 9 Network+ Certification, Second Edition 3. Windows 2000, Windows Me, and Windows 98 Second Edition include Internet Connection Sharing (ICS). a. ICS enables other computers on the local area network (LAN) to access the Internet through one computer’s dial-up connection to an Internet service provider (ISP). b. Some third-party software products provide Internet connection sharing capabilities. (1) These products are software routers that enable your computer to forward packets between the local network and the network run by your ISP. 4. Each network interface in an IP router must have its own IP address appropriate for the network it is attached to. C. Hardware routers 1. A stand-alone router is a hardware device that is essentially a special- purpose computer. 2. The router has multiple built-in network interface adapters, a processor, and memory in which it stores its routing information and temporary packet buffers. 3. Routers are available in a wide range of prices and with a variety of capabilities. 3. Routing Tables A. The routing table holds the information that the router uses to forward packets to the proper destinations. 1. Not only routers have routing tables; every Transmission Control Protocol/Internet Protocol (TCP/IP) system also has a routing table. |5| B. Direct route 1. A direct route is when a computer running TCP/IP transmits a packet to a destination on the local network. 2. In a direct route, the data-link layer Destination Address and the Destination IP Address in the IP header refer to the same computer. C. Indirect route 1. An indirect route is when a computer running TCP/IP transmits a packet to a destination on another network by forwarding the packet to a router on the local network. 2. In an indirect route, the Destination IP Address in the IP header refers to the packet’s ultimate destination and the Destination Address in the data- link layer protocol header refers to a router on the local network. 3. The system’s routing table contains the address of the router that the system should use to reach the destination. D. Routing table format ` 1. A routing table is a list of networks (and possibly hosts) and the addresses of routers that the system can use to reach them. 2. The arrangement of the information in the routing table can differ, depending on the operating system. |6| 3. Routing table columns and functions Outline, Chapter 9 3 Network+ Certification, Second Edition a. Network Address. Specifies the address of the network or host for which routing information is provided in the other columns b. Netmask. Specifies the subnet mask for the value in the Network Address column (1) As with any subnet mask, the system uses the Netmask value to determine which parts of the Network Address value are the network identifier, the subnet identifier (if any), and the host identifier. c. Gateway Address. Specifies the address of the router that the system should use to send datagrams to the network or host identified in the Network Address column (1) On a LAN, the hardware address for the system identified by the Gateway Address value will become the Destination Address value in the packet’s data-link layer protocol header. d. Interface. Specifies the address of the network interface adapter that the computer should use to transmit packets to the system identified in the Gateway Address column e. Metric. Contains a value that enables the system to compare the relative efficiency of routes to the same destination 4. Workstation Routing Table Entries A. First entry 1. The value 0.0.0.0 in the Network Address column identifies the default gateway entry. 2. The default gateway is the router on the LAN that the system uses when there are no routing table entries that match the Destination IP Address of an outgoing packet. 3. A routing table can have only one functional default gateway entry. 4. The Gateway Address column contains the IP address of a router on the local network. 5. The Interface column contains the IP address of the network interface adapter in the computer that connects the system to the network. B. Second entry 1. The value 127.0.0.0 in the Network Address column is the TCP/IP loopback address. a. IP automatically routes all packets destined for any address on the 127.0.0.0 network back to the incoming packet queue on the same computer. 2. The system uses its own loopback address (127.0.0.1) as the “router” to the destination. C. Third entry 1. The IP address of the network interface adapter for this computer is 192.168.2.2. 2. The value 192.168.2.0 in the Network Address column entry is the network address for the local network the computer is located on. a. The Network Address and Netmask values indicate that it is a Class C network. 4 Outline, Chapter 9 Network+ Certification, Second Edition 3. This is the entry the system uses for direct routes when it transmits packets to other systems on the local network. 4. The Gateway Address and Interface columns both contain the IP address of the network interface adapter for the computer, indicating that the computer should use itself as the gateway. D. Fourth entry 1. Routing tables can contain entries for host addresses as well as network addresses. 2. The value 192.168.2.2 in the Network Address column is the host address of the computer itself. 3. The system transmits data addressed to itself to the loopback address. 4. IP searches the routing table for host address entries before network address entries, so any packets addressed to 192.168.2.2 would use this entry before the third entry. E. Fifth and seventh entries 1. The addresses in the Network Address column are broadcast addresses. a. 255.255.255.255 is the generic IP broadcast address. b. 192.168.2.255 is the local network’s broadcast address. 2. The system uses itself as a gateway. F. Sixth entry 1. The value 224.0.0.0 is the network address for the multicast addresses designated by the Internet Assigned Numbers Authority (IANA) for specific purposes. |7| 5. Router Routing Table Entries A. The routing table on a router is considerably more complex than a workstation’s routing table because it contains 1. Entries for all of the networks that the router is attached to 2. Entries provided manually by administrators or dynamically by routing protocols B. Routers use the Interface and Metric columns more than workstations use them. 1. On a workstation with one network interface adapter, the Interface column is superfluous because there is only one interface to use. 2. Routers and multihomed systems have at least two network interfaces, so the value in the Interface column is a crucial part of transmitting a packet to the correct gateway. 3. On a singlehomed workstation, the Metric column is superfluous because the computer only has information about the local network. a. The Metric value for all of the entries is 1. |8| 6. Selecting a Table Entry A. IP uses the following procedure to determine the route for each packet it transmits: Outline, Chapter 9 5 Network+ Certification, Second Edition 1. After packaging the transport layer information into a datagram, IP compares the Destination IP Address for the packet with the routing table, looking for a host address with the same value. a. A host address entry in the table has a full IP address in the Network Address column and the value 255.255.255.255 in the Netmask column. 2. If there is no host address entry that exactly matches the Destination IP Address value, the system then scans the routing table’s Network Address and Netmask columns for an entry that matches the address’s network and subnet identifiers. a. If there is more than one entry in the routing table that contains the desired network and subnet identifiers, IP uses the entry with the lower value in the Metric column. 3. If no table entries match the network and subnet identifiers of the Destination IP Address value, the system searches for a default gateway entry that has a value of 0.0.0.0 in the Network Address and Netmask columns. 4. If there is no default gateway entry, the system generates an error message. a. If the system transmitting the datagram is a router, it transmits an Internet Control Message Protocol (ICMP) Destination Unreachable message back to the end system that originated the datagram. b. If the system transmitting the datagram is itself an end system, the error message gets passed back to the application that generated the data. 5. When the system locates a viable routing table entry, IP prepares to transmit the datagram to the router identified in the Gateway Address column. a. The system consults the Address Resolution Protocol (ARP) cache or performs an ARP procedure to obtain the hardware address of the router. 6. Once it has the router’s hardware address, IP passes it and the datagram down to the data-link layer protocol associated with the address specified in the Interface column. a. The data-link layer protocol constructs a frame using the router’s hardware address in its Destination Address field and transmits it out over the designated interface. Chapter 9, Lesson 2 Building Routing Tables |9| 1. Static and Dynamic Routing A. The two techniques for updating the routing table are static routing and dynamic routing. B. Static routing 1. Process by which administrators create routing table entries manually, using a program designed for that purpose 6 Outline, Chapter 9 Network+ Certification, Second Edition 2. Effective on small networks that never change or that have only one route to each destination C. Dynamic routing 1. Process of creating routing table entries automatically by using specialized routing protocols that run on router systems, such as a. Routing Information Protocol (RIP) b. Open Shortest Path First (OSPF) protocol 2. Routers use these protocols to exchange messages containing routing information with other nearby routers. 3. Advantages: a. Reduces the administrative workload b. Automatically compensates for changes in the network infrastructure 4. Dynamic routing is essential on the Internet. |10| 2. Creating a Static Route A. Static routing programs 1. UNIX uses route. 2. Windows uses ROUTE.EXE. B. Using ROUTE.EXE |11| 1. ROUTE.EXE syntax: ROUTE [-f] [-p] [command [destination] [MASK netmask] [gateway] [METRIC metric] [IF interface]] 2. ROUTE.EXE parameters a. -f This parameter deletes all of the entries from the routing table. (1) When used with the ADD command, it deletes the entire table before adding the new entry. b. -p When used with the ADD command, this parameter creates a persistent route entry in the table. (1) A persistent route is one that remains in the table permanently, even after the system is restarted. (2) When -p is used with the PRINT command, the system displays only persistent routes. c. command This variable contains a keyword that specifies the function of the command. d. destination This variable specifies the network or host address of the table entry being managed. e. MASK netmask The variable netmask specifies the subnet mask to be applied to the address specified by the destination variable. f. gateway This variable specifies the address of the router that the system should use to reach the host or network specified by the destination variable. g. METRIC metric The variable metric specifies a value that indicates the relative efficiency of the route in the table entry. Outline, Chapter 9 7 Network+ Certification, Second Edition h. IF interface The variable interface specifies the number of the network interface adapter that the system should use to reach the router specified by the gateway variable. |12| 3. ROUTE.EXE command variable options a. PRINT. Displays the contents of the routing table (1) When used with the -p parameter, it displays only the persistent routes in the routing table. b. ADD. Creates a new entry in the routing table c. DELETE. Deletes an existing entry from the routing table d. CHANGE. Modifies the parameters of an entry in the routing table |13| 4. ROUTE.EXE example: ROUTE ADD 192.168.5.0 MASK 255.255.255.0 192.168.2.7 IF 1 METRIC 1 a. ADD. Indicates that the program should create a new entry in the existing routing table b. 192.168.5.0. The address of the other network to which Router B provides access c. MASK 255.255.255.0. The subnet mask to be applied to the destination address, which in this case indicates that the address represents an unsubnetted Class C network d. 192.168.2.7. The address of the network interface adapter with which Router B is connected to the same network as Router A e. IF 1. The number of the network interface adapter in Router A that provides access to the network it shares with Router B f. METRIC 1. Indicates that the destination network is one hop away |14| g. This routing table entry tells Router A that when it has traffic to send to any computer on the network with the address 192.168.5.0, it should send the traffic to the router with the address 192.168.2.7, using the Router A network interface adapter with the interface number 1. |15| C. Routing and Remote Access 1. Included in Microsoft Windows 2000 Server 2. Available as a free add-on for Windows NT Server 4.0 3. Expands the routing capabilities of the operating system by supporting the following features: a. RIP version 2 b. OSPF c. ICMP router discovery d. Demand dialing e. Point-to-Point Tunneling Protocol (PPTP) f. Graphical interface access to the routing table 3. Dynamic Routing A. Sharing routing table information 1. Routers only have direct knowledge of the networks they are connected to. 8 Outline, Chapter 9 Network+ Certification, Second Edition 2. When there are two or more routers on an internetwork, dynamic routing a. Enables each of the routers to know about the others b. Creates routing table entries that specify the networks that the other routers are connected to |16| 3. Router A can have direct knowledge of Router B from routing protocol broadcasts, because both are connected to the same network. 4. Router B has knowledge of Router A for the same reason, but it also has knowledge of Router C, because Router C is on another network that Router B is connected to. 5. Router A has no direct knowledge of Router C, because they are in different broadcast domains. a. By using a dynamic routing protocol, Router B can share its knowledge of Router C with Router A, enabling A to add C to its routing table. 6. By using a routing protocol to share the information in their routing tables, routers obtain information about distant networks and can route packets more efficiently. B. The TCP/IP suite includes many routing protocols. 1. On a private internetwork, one routing protocol, such as RIP, is usually sufficient to keep all of the routers updated. 2. On the Internet, routers use various protocols, depending on their place in the network hierarchy. |17| 3. Routing protocols are generally divided into two categories: a. Interior gateway protocols (IGPs). Routing protocols used by routers that are all within the same autonomous system (AS) b. Exterior gateway protocols (EGPs). Used for communications between autonomous systems C. RIP |18| 1. RIP characteristics a. RIP is the acronym for Routing Information Protocol. b. Most common IGP in the TCP/IP suite c. Originally designed for UNIX systems as a daemon called routed d. Eventually ported to other platforms e. Standardized in Request for Comments (RFC) 1058 f. Updated to version 2, published as RFC 2453 2. RIP communications a. RIP uses request and reply message types. b. All RIP messages are carried in User Datagram Protocol (UDP) packets addressed to well-known port number 520. |19| c. RIP routers initiate communications when starting up, by broadcasting a request message on all network interfaces. d. All RIP routers receiving the broadcast respond with reply messages containing their entire routing table. e. The router receiving the replies updates its own routing table with the information in the reply messages. Outline, Chapter 9 9 Network+ Certification, Second Edition |20| f. RIP reply messages can contain up to 25 routes, each of which is 20 bytes long. (1) Routers use multiple messages when necessary. g. RIP routers transmit their routing tables every 30 seconds. h. If a RIP-supplied routing table entry is not refreshed every 3 minutes, the router stops using it. 3. RIP metrics a. RIP uses the number of hops to the destination to determine a route’s efficiency. b. When routers receive routing table entries in RIP messages, they increment the value of the metric in each route to reflect the additional hop required to reach the destination. c. The maximum value for a metric in a RIP message is 15. d. Routing that uses metrics based on the number of hops to the destination is called distance vector routing. 4. RIP drawbacks a. The protocol generates large amounts of broadcast traffic. b. The RIP version 1 message format does not include a subnet mask for each route. 5. RIP version 2 a. The RIP version 2 message format is the same size as RIP version 1, but it uses the unused fields to include additional information about each route. b. RIP version 2 supports the use of multicast transmissions, which reduces the amount of traffic generated by the protocol. |21| 6. RIP version 2 message format a. Address Family Identifier (2 bytes). Contains a code that identifies the protocol for which routing information is being provided. (1) The code for IP is 2. (RIP supports other protocols besides IP.) b. Route Tag (2 bytes). Contains an autonomous system number that enables RIP to communicate with EGPs c. IP Address (4 bytes). Specifies the address of the network or host for which routing information is being provided d. Subnet Mask (4 bytes). Contains the subnet mask that the router should apply to the IP Address value e. Next Hop IP Address (4 bytes). Specifies the address of the gateway that the router should use to forward traffic to the network or host specified in the IP Address field f. Metric (4 bytes). Contains a value that specifies the relative efficiency of the route |22| D. OSPF 1. OSPF is the acronym for Open Shortest Path First protocol. 2. Standardized in RFC 2328 3. Uses link-state routing 10 Outline, Chapter 9 Network+ Certification, Second Edition a. Judging routes by the number of hops required to reach a destination can be inefficient because a hop can refer to a connection of any speed. b. Link-state routing measures the actual properties of each connection and stores the information in a database that is shared among the routers on the network. 4. Advantages of OSPF a. Updates routing tables more quickly when changes occur on the network (called convergence) b. Can balance the network load by splitting traffic between routes with equal metrics c. Supports authentication of routing protocol messages |23| Chapter Summary A. Routing principles 1. Routers receive packets and use the most efficient path to forward them to their destinations. 2. Complex internetworks can have redundant routers that provide multiple paths to the same destination. 3. Routers store information about the network in a routing table. B. Building routing tables 1. Information gets into the routing table in one of two ways: a. Static routing, which is the manual creation and maintenance of table entries b. Dynamic routing, which uses specialized routing protocols to update the routing table 2. The Windows 2000 ROUTE.EXE program provides direct access to the routing table. 3. Dynamic routing enables routers to share the information in their tables with the other routers on the network.