After completing this lesson, you will be able to do the following: Describe the basic process of IP routing.

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1 Lesson 3-2: Routing At a Glance Routers perform many functions on a network, from segmenting large networks so that they work more efficiently to interconnecting networks that use different LAN or WAN transmission technologies. The job that routers are best know for, however, is forwarding data from a host on one network to a host on another network. Whether the data is an message, a huge video file, or a simple ping, a router will read the data s destination, determine how it should get there, and send it onward. This lesson will introduce the basics of the routing procedure and examine a typical data packet and a routing table. What You Will Learn After completing this lesson, you will be able to do the following: Describe the basic process of IP routing. Explain the process by which routers forward packets. Describe the components of a Nortel Networks routing table. Explain the difference between dynamic and static routing table. Configure a static route. ST A 137

2 Lesson 3-2: Routing 138 ST A Routing

3 Tech Talk Datagram A packet of data at the network layer. Frame A packet of data at the data link layer. Packet A package of data transmitted over a communications link. Time to Live The amount of time in seconds that a packet is allowed to try to reach its destination before being thrown away. Hop A jump that a datagram takes from one router to the next. Routing Table A table that tells a router the next hop a packet should take toward its destination. Gateway Router A router that functions as the connection between a network and the Internet. Static Route An entry in a routing table manually entered by a network administrator. Default Route An entry in a routing table that directs a router where to send packets for which there is no other table entry. ST A 139

4 Lesson 3-2: Routing Basic Router Operation Routers find paths to forward data packets from a device on one network to a device that may be on a network nearby or far away. Routers discover paths, figure out which is the best one to use, and remember that information in a routing table. This ability sets routers apart from bridges. Routers are intelligent and they use this intelligence to find routes. A router listens to the LAN transmission media and accepts frames that are addressed to a port on the router or that are addressed to the entire network (called a broadcast address). Once it receives all the that make up a frame, the router, operating at Layer 2, strips off the framing that was added by the LAN protocol (for example, Ethernet) to reveal the IP packet inside. The router performs three basic operations on the IP packet. Operating at Layer 3, the router uses the IP protocol to get the destination address from the IP packet. The router looks up that address in its routing table. If the router finds the destination address in its routing table, it looks up which of its ports is connected to the machine or network with that address. If the router checks its routing table and does not find the destination address of the packet then it looks up which of its ports is connected to the default route. Then, the IP protocol switches the packet to the port connected to the destination or the port connected to the default route. If the router has not found the destination address in its routing table and does not have a default route defined, it discards the packet. 140 ST A Routing

5 IP Datagrams Routers forward data in the form of IP packets, which are more appropriately called datagrams. The most important parts of an IP datagram are the data it contains and the source and destination address. The addresses are the IP addresses that you studied in the previous lesson. The table below includes a complete list of all the parts of an IP datagram header. IP Datagram Header 4 4 8x x Var. IP Version * Header Length Service Flags Total Length Packet ID Flags Fragment Offset Time to Live Protocol IP Header Checksum Source IP Address Destination IP Address Padding Data IP Version There are two versions of IP: IPv4 and IPv6. All nodes and gateways must agree on the version to use. Header Length The length of the header. Service Flags These flags can be used to specify special treatment for the packet. Total Length The total length of the packet in octets. Packets can be up to 65,535 octets long. Packet ID For faster transmission, sometimes a datagram is broken into fragments. A packet identifier is assigned to each fragment so that they can be reassembled correctly. Flags These three are used to manage fragmentation of a datagram. The first bit is always 0. The second bit is used by the sending node. It is 1 if the datagram is fragmented and 0 if it is not. The third bit is used by the receiving node. It is 1 if there are more fragments following this one or 0 if this is the last fragment. Fragment Offset This number tells where the fragment fits in the reassembled packet. ST A 141

6 Lesson 3-2: Routing Time To Live This specifies how many seconds the packet can remain in the Internet. Routers that forward a packet will decrease this time to live counter by one. If the counter reaches zero, the packet is discarded and an Internet Control Message Protocol (ICMP) message is sent back to the source host. Time to Live ensures that packets that cannot get to their destinations do not hang around and clog up the Internet. Protocol ID Specifies the upper layer protocol, such as TCP or UDP, which is encapsulated in the packet. Header Checksum Used to verify that the header (not the actual data, just the header) has not been corrupted during transmission. Source IP Address The address of the machine that sent the packet. Destination IP Address The address of the machine to which the packet is going. Padding Extra zeroes are added to the packet so that its length is a multiple of 32. Data This portion of the datagram is variable in length to accommodate the actual information being transmitted. The data carried by an IP datagram is most often contained in a TCP segment or a UDP datagram. These two data structures are important to recognize for what they have in common and for how they are different. They are similar in that both contain an even more specific address than the destination IP address of the datagram. TCP and UDP addresses include a port number that identifies a particular process on the host. Port numbers are not standardized but usually a particular process will have the same port number on most hosts. For example, a web server usually has a port number of 80. TCP and UDP are different in that TCP is connection-oriented while UDP is connectionless. Routing Tables When a datagram arrives at the router on its way to another host, the router looks up that destination in its routing table, finds the correct port for that destination, and sends the packet out via that port. A routing table contains entries that specify the next hop in the transmission path a packet must follow to get to a particular destination address. If the router had to keep a table of routes to every computer on the Internet, it would have to hold millions and millions of possible routes. This would cause several major problems. The router would need a huge amount of memory to store the table with all the routes. 142 ST A Routing

7 Each time a packet arrived, the router would have to search through a huge table to find the best route for that packet. The router would have to spend a lot of time checking those possible routes to make sure they were open. Scalability is the problem of how to make routers continue to work quickly even as the number of destinations grows. As you learned in the IP Addressing lesson, to solve the scalability problem, the job of routing is distributed among routers. Instead of remembering the address and the route to every host on the Internet, some routers remember only the address and the route to other networks. Once the packet gets to the correct network, a router on that network sends the packet to the correct host. This way, a router in one part of the Internet does not have to know the route to every machine on a network in another part of the Internet. It only has to know how to get a packet to that network. In fact, the router does not necessarily need to know how to get the packet all the way to the other network. It only needs to know how to send the packet toward its destination. Check Your Understanding List three possible problems that might occur if a router had to keep all the addresses of all the computers attached to the Internet. ST A 143

8 Lesson 3-2: Routing Routing Table Fields A routing table has several fields. The routing table for a Nortel Networks router has seven fields. Destination The dotted decimal address of the destination network. Metric Cost to the destination network. Depending on the protocol that learned the route, this may be a simple hop count or a user assigned cost value. Next Hop The dotted-decimal IP address of the next hop interface used to forward a packet through the network. Type The type of route. Direct indicates that the destination network for this route is directly connected to the router. Indirect indicates that the destination network is not directly connected to the router. Protocol The way the router learned this route. Routers have the ability to learn routes on their own. Age The number of seconds since this route was last updated. Index This number is assigned by the router to the circuit over which the next hop can be reached. 144 ST A Routing

9 How a Router Consults a Routing Table Router A IP Diagram Table A 1 B 1 C 2 D 3 E 1 F 2 G 3 H 3 Port 1 Port 2 Port 3 B Router C Router E Router F Router Router D Router G Router H Router When a router receives an IP datagram, it checks the destination address and consults the routing table to find a next hop that is associated with that address. The next hop address is the critical piece of information that a router must contain. For destination machines that are connected to the same network as the router, the routing table includes the Layer 3 address (the IP address) of the machine and the number of the port on the router that it is connected to. For destination machines or networks that are not directly connected to the router, the table includes the Layer 3 address of the machine and the number of the port on the router that will send the data in the right direction. Cost is another critical piece of information. It specifies the number of router hops a datagram can traverse before reaching the destination IP address. The router uses the cost value when determining the best route for a datagram to follow. ST A 145

10 Lesson 3-2: Routing Static Routing A router acquires routes in two primary ways: Through dynamic routing, the router can use any of a number of protocols to learn routes. Through static routing, a network administrator can enter the routes directly into the table. A network administrator might define a static route to: Control the path that a datagram follows. Decrease the amount of traffic exchanged between routers. In future lessons you will learn that routers must communicate with other routers about routes and the status of those routes. This communication can take up valuable bandwidth. With a static route, updates do not need to be broadcast to other servers as frequently. Define a default route. Improve network security. The route into and out of a network can be limited to one predefined path. Improve the efficiency of the network. When a router is given the route to use, it does not have to make calculations, which speeds up its performance. Using a limited number of static routes, a router also does not need to store the incoming data while it figures out where it should go, it already knows the correct route when it receives the data. This means the router needs less buffer memory. When configuring static routes, consider the following: A static route can be assigned a preference from 1 to 16, with 16 being the most preferred. When the router must choose between multiple routes to the same destination, it will choose the route with the highest preference. Routing protocols such as RIP assign the routes they discover a routing preference of 1. Static routes remain in IP routing tables until you remove them. Note, however, that if the interface that was used to reach the next hop in the static route becomes disabled, the static route disappears from the IP routing table. 146 ST A Routing

11 Static Route Example 1 In the example below, there are two connections between Router A and Router B: a Mbps T1 line and a backup connection that uses one channel on a dial-up ISDN line. The path a datagram will take to get from host A to host B cannot be predicted. The path can be controlled so that the T1 line is used all the time and the dial-up line is reserved for emergencies: 1. At router A, build a static route: a. Destination Network b. Next Hop Address c. Preference At router C, build a static route: a. Destination Network b. Next Hop Address c. Preference 16. Static Route Example Host A.1.1 Router A.1 Host B K.1 T Router C Network Subnet Mask = ST A 147

12 Lesson 3-2: Routing Static Route Example 2 Static routes can be used to override the routes that routing protocols mistakenly select. In the second example, IP communication between host A and host C might occur over the 64K synchronous link because it has the lowest cost, that is, the fewest hops. But in this case, the T1 links would have a higher bandwidth even though they have an additional hop. It is possible to force Router A to connect to Router C via the T1 by configuring a static route in the router table of each router as follows: 1. At router A, build a static route: a. Destination Network b. Next Hop Address c. Preference At router B, build two static routes: d. Destination Network e. Next Hop Address f. Preference 16. g. Destination Network h. Next Hop Address i. Preference At router C, build a static route: j. Destination Network k. Next Hop Address l. Preference 16. The significant problem with static routes is that they must be planned carefully. If they are not, the network administrator may need to reconfigure the routing table whenever a failure occurs. 148 ST A Routing

13 Static Route Example Host A K Host B.1.1 Router A. 1 T Router B. 1 T Router C Network Subnet Mask = Because there is an additional hop in this diagram, the simple static route from the previous example will not be sufficient in case of a failure. If the T1 connected to router A fails, Router A will fall back on the route over the 64K line. Router C, however, will still have a viable static route with a high preference and would not switch over to the 64K line unless there were another static route for the lower preference 64K line. Then once router A failed over to the 64K line and router C received ICMP messages stating that the destination network over the T1 line was unavailable it too would switch over to the 64K ISDN backup link. Another solution to the stated problem would be through use of a static route as follows: Define a static route to network on Router A, using a preference of 1 and a next hop address of Define a static route to network on Router C, using a preference of 1 and a next hop address of ST A 149

14 Lesson 3-2: Routing Default Route The address is used to point to a default gateway. It can be used: On a station for directing communications to nodes not on the same physical network. On a router to direct requests for unknown networks to a gateway router. Example: Defining a Default Route Address is used to signify a default route. If it is in a routing table, it can be interpreted to mean any network to which the path is not known. Therefore, it is possible to define a static default route to point to some type of gateway router. This gateway router might be the entry point to the Internet. Defining a default route allows a router to still forward packets even though it does not have any routing information about the destination network. Default Route Example Host A Host B 64K Router A.1.1 T T Router C Router B.4 Gateway Static Route Definition (Default Route) Destination Address: Subnet Mask: Next Hop Address: Next Hop Mask: Network Subnet Mask = ST A Routing

15 Dynamic Routing The key to routing lies in routers ability to build and maintain their own routing tables. As a router monitors all the packets that are being transmitted on the networks attached to its ports, it learns about the network and host IP addresses that can be reached from each port. In its routing table the router keeps track of these addresses and the ports they are connected to. Routing is a complex job. There is usually more than one way to get a packet from its source to its destination, routers must determine which route is best. As machines are added or removed from networks, routers must keep track of which ones are there and how to get to them. As new networks are created or subdivided from other networks, routers must keep track of which addresses belong to which networks and how to send data to those addresses. As paths between networks get congested or clear up, routers must keep track of their status so the fastest route can always be found. Routing Protocols Routers use routing protocols to perform the tasks described above. Such routing protocols as RIP and OSPF allow routers to find available routes, to communicate to other routers what those routes are and their status, to select the best route for any particular packet, and to send data packets along that route. Routers use routing protocols to: Discover routes to specific destinations. Calculate the best route to a specific destination. Monitor the network for changes or interruptions in the routes. Communicate information about routes to other routers. Check Your Understanding Briefly distinguish between a static routing table and a dynamic routing table. ST A 151

16 Lesson 3-2: Routing Try it Out Adding a Static Route Site Manager allows a network administrator to add a static route to the ARN. Materials Needed: Classroom Network Windows 95 PC Site Manager Any Word Processor (e.g., MS Word) Pen/Pencil and Paper Student Portfolio In this lab you will learn how to: Configure a static route. During this lab, work in teams of three. Record your experiences, results, speculations, and conclusions in your portfolio. 1. In the Configuration Manager window in Site Manager, click Protocols. 2. Click IP. 3. Click Static Routes. 4. Click Add. 152 ST A Routing

17 The table below describes the parameters to add a static route. Parameter Default Options Destination IP Address None Any valid IP network address Address Mask None Based on the network class of the IP address you specified at the Destination IP Address parameter. Cost (Specifies the number of router hops that will be traversed before reaching the destination IP address. This cost is also communicated to other routers by a routing protocol). Next Hop Address (Specifies the address of the next-hop router). Next Hop Mask (Specifies the subnet mask of the next hop router). Preference (Specifies a weighted value (from 1 to 16, with 16 being the most preferred) that the IP router uses to select a route when its routing table contains multiple routes to the same destination.) 1 1 to the value of the RIP Diameter parameter Any valid IP address Any valid subnet mask address to 16 ST A 153

18 Lesson 3-2: Routing 5. In the Configuration Manager window, enter values from your classroom network topology map for: a. Destination b. IP Address. c. Address Mask. d. Next Hop Addr. e. Next Hop Mask. Rubric: Suggested Evaluation Criteria and Weightings Criteria % Your Score Complete record of procedural results. 25 Summary, analysis, synthesis and conclusions 50 Organization and summary in format suitable for reproduction 25 TOTAL ST A Routing

19 Stretch Yourself Comparing Datagrams Materials Needed: Windows 95 PC Internet Connection Sniffer Basic Software Any Word Processor (e.g., MS Word) Pen/Pencil and Paper Student Portfolio 1. Use Snifffer Basic to examine the difference between an IP datagram containing a TCP segment and one containing a UDP datagram. 2. Monitor the IP Protocols while operating a process that sends a TCP segment (i.e., Telnet) and one that sends a UDP datagram (i.e., SNMP). 3. Record your observations. 4. In addition to differences you notice between TCP and UDP, be sure to also note the following: a. How did you determine which application uses TCP and which uses UDP? b. Why do you think the applications have been set up this way? 5. Participate in a class discussion on the results of this activity. Rubric: Suggested Evaluation Criteria and Weightings Criteria % Your Score Successful completion of the activity. 20 Analysis and synthesis of information. 50 Insightful and enthusiastic participation in class discussion. 30 TOTAL 100 ST A 155

20 Lesson 3-2: Routing Network Wizards VisualRoute VisualRoute is an inexpensive software package that allows the user to trace the route of a packet from its source to a specified destination, such as a web site s router. There is a demo version available on Data Metric s web site that allows the user to try out the product. This activity uses the demo version only. Materials Needed: Windows 95 PC Internet Connection VisualRoute Software Demo downloaded Any Word Processor (e.g., MS Word) (optional) Pen/Pencil and Paper Student Portfolio 1. Access the demo version by entering into your web browser. 2. To first experience the demo, enter the Nortel Networks URL, into the Enter Host/URL box. 156 ST A Routing

21 3. Note the route the packet took from your workstation to Nortel Networks. ST A 157

22 Lesson 3-2: Routing 4. Now find a destination that takes only a few hops. 5. Note the path of the packet. 6. Find a destination that takes more than 25 hops. 7. Note the path of the packet. 8. Pick one of the traces, and make a screen shot (Alt + PrntScrn) for your portfolio. 9. Analyze the IP address of each hop. What the class is of each hop. Are some hops on the same network? Speculate why, in some cases, it takes the packet longer to get from one hop to another. 10. In your portfolio, write a summary of this activity, including the answers to step 9. Rubric: Suggested Evaluation Criteria and Weightings Criteria % Your Score Thorough summary in portfolio 50 Correct identification of IP classes 50 TOTAL 100 Summary In this unit, you learned the following: The routing function of the IP protocol stack. The components of an IP packet header. The process by which routers forward packets. The components of a Nortel Networks routing table. The difference between dynamic and static routes. How to configure and edit a static route. 158 ST A Routing

23 Review Questions Name Lesson 3-1: Routing Part A 1. Describe the three basic operations a router performs on an IP packet. Part B 1. Put the following steps in the IP routing sequence into order a. Receive frame b. Check destination IP address against routing table c. Switch packet to correct port d. Put packet into frame e. Strip frame f. Check MAC address 2. What does a router do with a packet when it can t find the destination address in its router table? a. Discard it b. Send it to the default route c. Store it until it s told the route d. Send it to a static route e. a or b f. c or d ST A 159

24 Lesson 3-2: Routing Part C 1. Describe the components of a Nortel Networks routing table. Part D 1. Explain one advantage and one disadvantage to static routing. 2. Describe the difference between a dynamic and a static routing table. Scoring Rubric: Suggested Evaluation Criteria and Weightings Criteria % Your Score Part A: Describe the basic process of IP routing. Part B: Explain the process by which routers forward packets. Part C: Explain the components of a Nortel Networks routing table. Part D: Explain the difference between dynamic and static routes TOTAL 100 Try It Out: Configure a static route. 100 Stretch Yourself 100 Network Wizards 100 FINAL TOTAL ST A Routing

25 Resources Bay Networks. (1999). Accelerated Router Configuration, Bay Networks, Inc., Billerica, Massachusetts. ST A 161