Cisco ICND1 Section 3 IP Routing

1. How does Cisco IOS Software designate a subnet in the routing table as a directly connected network? What about a route learned with RIP?

1. Answer: The show ip route command lists routes with a designator on the left side of the command output. C represents connected routes, I is used for IGRP, and R represents routes derived from RIP.

10. Name three possible problems that could prevent the command boot system tftp c2500-j-l.112-14.bin 128.1.1.1 from succeeding.

10. Answer: The possible reasons include: 128.1.1.1 is not accessible through the network, there is no TFTP server on 128.1.1.1, the file is not in the TFTP default directory, the file is corrupted, or a different boot command could precede this boot command in the configuration file, meaning that the IOS referenced in the first boot command would be used instead.

11. Two different Cisco IOS files are in a router's Flash memory: one called c2500-j-l.1113.bin and one called c2500-j-l.112-14.bin. Which one does the router use when it boots up? How could you force the other Cisco IOS file to be used? Without looking at the router configuration, what command could be used to discover which file was used for the latest boot of the router?

11. Answer: The first Cisco IOS file listed in the show flash command is the one used at reload time, unless a boot system command is configured. The configuration command boot system flash c2500-j-l.112-14.bin would override the IOS's decision Part III: IP Routing—Answers 27 to look for files in order in Flash memory. show version is the command used to display the filename of IOS for the latest reload of a router. The show version output tells you the version as well as the name of the file that was used at last reload time. It is particularly difficult to find in the output of the command.

12. Name the parts of an IP address.

12. Answer: Network, subnet, and host are the three parts of an IP address. However, many people commonly treat the network and subnet parts of an address as a single part, leaving only two parts, the subnet and host parts. On the exam, the multiple-choice format should provide extra clues as to which terminology is used.

13. Define the term subnet mask. What do the bits in the mask whose values are binary 0 tell you about the corresponding IP address(es)?

13. Answer: A subnet mask defines the number of host bits in an address. The bits of value 0 define which bits in the address are host bits. The mask is an important ingredient in the formula to dissect an IP address; along with knowledge of the number of network bits implied for Class A, B, and C networks, the mask provides a clear definition of the size of the network, subnet, and host parts of an address.

14. Given the IP address 134.141.7.11 and the mask 255.255.255.0, what is the subnet number?

14. Answer: The subnet is 134.141.7.0.

15. Given the IP address 193.193.7.7 and the mask 255.255.255.0, what is the subnet number?

15. Answer: The network number is 193.193.7.0. Because this is a Class C address and the mask used is 255.255.255.0 (the default), no subnetting is in use.

16. Given the IP address 200.1.1.130 and the mask 255.255.255.224, what is the subnet number?

16. Answer: The answer is 200.1.1.128. Calculate the subnet number without binary math. The magic number is 256 - 224 = 32.

17. Given the IP address 220.8.7.100 and the mask 255.255.255.240, what is the subnet number?

17. Answer: The answer is 220.8.7.96. Calculate the subnet number without binary math. The magic number is 256 - 240 = 16.

18. Given the IP address 134.141.7.11 and the mask 255.255.255.0, what is the subnet broadcast address?

18. Answer: The broadcast address is 134.141.7.255.

19. Given the IP address 193.193.7.7 and the mask 255.255.255.0, what is the broadcast address?

19. Answer: The broadcast address is 193.193.7.255. Because this is a Class C address and the mask used is 255.255.255.0 (the default), no subnetting is in use.

2. Imagine that a router has an interface E0 with IP address 168.10.1.1 and an interface E1 with IP address 10.1.1.1. If the commands router rip and network 10.0.0.0, with no other network commands, are configured in the router, does RIP send updates out E0?

2. Answer: No. There must be a network statement for network 168.10.0.0 before RIP advertises out that interface. The network command simply selects the connected interfaces on which to send and receive updates.

20. Given the IP address 200.1.1.130 and the mask 255.255.255.224, what is the broadcast address?

20. Answer: The broadcast address is 200.1.1.159.

21. Given the IP address 220.8.7.100 and the mask 255.255.255.240, what is the broadcast address?

21. Answer: The broadcast address is 220.8.7.111.

22. Given the IP address 134.141.7.11 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

22. Answer: The subnet number is 134.141.7.0, and the subnet broadcast address is 134.141.7.255. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 134.141.7.1 to 134.141.7.254.

23. Given the IP address 193.193.7.7 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

23. Answer: The subnet number is 193.193.7.0, and the network broadcast address is 193.193.7.255. The assignable addresses are all the addresses between the network and broadcast addresses, namely 193.193.7.1 to 193.193.7.254.

24. Given the IP address 200.1.1.130 and the mask 255.255.255.224, what are the assignable IP addresses in this subnet?

24. Answer: The subnet number is 200.1.1.128, and the subnet broadcast address is 200.1.1.159. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 200.1.1.129 to 200.1.1.158.

25. Given the IP address 220.8.7.100 and the mask 255.255.255.240, what are the assignable IP addresses in this subnet?

25. Answer: The subnet number is 220.8.7.96, and the subnet broadcast address is 220.8.7.111. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 220.8.7.97 to 220.8.7.110.

26. Given the IP address 134.141.7.7 and the mask 255.255.255.0, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

26. Answer: The answer is 134.141.0.0 (zero subnet), 134.141.1.0, 134.141.2.0, 134.141.3.0, and so on, up to 134.141.254.0 and 134.141.255.0 (broadcast subnet).

27. Given the IP address 220.8.7.100 and the mask 255.255.255.240, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

27. Answer: The answer is not as obvious in this question. The Class C network number is 220.8.7.0. The mask implies that bits 25 through 28, which are the first 4 bits in the fourth octet, comprise the subnet field. The answer is 220.8.7.0 (zero subnet), 220.8.7.16, 220.8.7.32, 220.8.7.48, and so on, through 220.8.7.240 (broadcast subnet).

28. How many IP addresses could be assigned in each subnet of 134.141.0.0, assuming that a mask of 255.255.255.0 is used? If the same (static) mask is used for all subnets, how many subnets are there?

28. Answer: There will be 2 hostbits, or 2 to the 8th hosts per subnet, minus two special cases. The number of subnets will be 2 subnetbits, or 2 to the 8th.

29. How many IP addresses could be assigned in each subnet of 220.8.7.0, assuming that a mask of 255.255.255.240 is used? If the same (static) mask is used for all subnets, how many subnets are there?

29. Answer: There will be 2 hostbits, or 2 to the 4th hosts per subnet, minus two special cases. The number of subnets will be 2 subnetbits, or 2 to the 4th.

3. Which command lists all the IP routes learned via RIP?

3. Answer: The show ip route rip command lists only RIP-learned routes.

30. You design a network for a customer, and the customer insists that you use the same subnet mask on every subnet. The customer will use network 10.0.0.0 and needs 200 subnets, each with 200 hosts maximum. What subnet mask would you use to allow the largest amount of growth in subnets? Which mask would work and would allow for the most growth in the number of hosts per subnet?

30. Answer: Network 10.0.0.0 is a Class A network, so you have 24 host bits with no subnetting. To number 200 subnets, you will need at least 8 subnet bits because 28 is 256. Likewise, to number 200 hosts per subnet, you will need 8 host bits. So, you need to pick a mask with at least 8 subnet bits and 8 host bits. 255.255.0.0 is a mask with 8 subnet bits and 16 host bits. That would allow for the 200 subnets and 200 hosts, while allowing the number of hosts per subnet to grow to 216 - 2, quite a large number. Similarly, a mask of 255.255.255.0 gives you 16 subnet bits, allowing 216 subnets, each with 28 - 2 hosts per subnet.

31. What are the valid private IP network numbers, according to RFC 1918?

31. Answer: Network 10.0.0.0, Class B networks from 172.16.0.0 through 172.31.0.0, and Class C networks beginning with 192.168.

32. How large are IPv6 addresses?

32. Answer: 128 bits long.

33. How does CIDR help reduce the size of Internet routing tables?

33. Answer: By using a routing protocol that exchanges the mask as well as the subnet/network number, a classless view of the number can be attained. By advertising many networks as a single route, the routing table can be shortened. For instance, 198.0.0.0/8 (198.0.0.0, mask 255.0.0.0) defines a set of addresses whose first 8 bits are equal to decimal 198. Instead of the more than 65,000 routes needed to list a route for each Class C network that starts with 198, CIDR allows those routes to be represented by a single route.

34. Create a minimal configuration enabling IP on each interface on a 2501 router (two serial, one Ethernet). Use Class A network 8.0.0.0. Your boss says that you need, at most, 200 hosts per subnet. You decide against using VLSM. Your boss also says to plan your subnets so that you can have as many subnets as possible rather than allow for larger subnets later. When choosing the actual IP address values and subnet numbers, you decide to start with the lowest numerical values. Assume that point-topoint serial links will be attached to this router. Avoid using the zero subnet.

34. Answer: interface ethernet 0 [ENTER], ip address 8.0.1.1 255.255.255.0 [ENTER], interface serial 0 [ENTER], ip address 8.0.2.1 255.255.255.0 [ENTER], interface serial 1 [ENTER], ip address 8.0.3.1 255.255.255.0 [ENTER]. The zero subnet was not used in this solution. If desired, the ip subnet-zero global command could have been used, enabling subnet 8.0.0.0/24 as well as subnets 8.0.1.0/24 and 8.0.2.0/24 to be used as the three subnets in the configuration.

35. In the previous question, what would be the IP subnet of the link attached to serial 0? If another user wanted to answer the same question but did not have the enable password, what command(s) might provide this router's addresses and subnets?

35. Answer: The attached subnet is 8.0.2.0, 255.255.255.0. The show interface, show ip interface, and show ip interface brief commands would supply this information, as would show ip route. The show ip route command would show the actual subnet number instead of the address of the interface.

36. What must be done to make the output of the show ip route command list subnet masks in decimal format instead of prefixes? In what mode would you use the command?

36. Answer: You must use the terminal ip netmask-format decimal command in EXEC mode.

37. In the output of show ip route, when a C shows up in the left side of the output on a line for a particular route, what does that mean?

37. Answer: C means "connected." This means that the route was learned by this router because it has an operational interface that is connected directly to that subnet.

38. Define the term prefix notation. Give two examples.

38. Answer: The representation of a subnet mask with a slash character (/), followed by a number that represents the number of binary 1s in the mask. For example, /16 means the same thing as 255.255.0.0, and /22 means the same thing as 255.255.252.0.

39. What does ICMP stand for? To which OSI layer would you consider this protocol to apply most closely?

39. Answer: Internet Control Message Protocol. ICMP is considered a Layer 3 protocol because it is used for control and management of IP.

4. Which command or commands list all IP routes in network 172.16.0.0?

4. Answer: The show ip route 172.16.0.0 command lists all the routes in 172.16.0.0. Also, the show ip route list 1 command lists routes in network 172.16.0.0 assuming that the access-list 1 permit 172.16.0.0 0.0.255.255 configuration command also exists.

40. Identify two methods to tell a router to ask for name resolution from two different name servers.

40. Answer: ip name-server 1.1.1.1 2.2.2.2 [ENTER], or ip name-server 1.1.1.1 [ENTER], ip name-server 2.2.2.2. Both methods tell the router to first try the name server at 1.1.1.1 and then try the name server at 2.2.2.2.

41. What keyboard sequence suspends a Telnet session in a Cisco router?

41. Answer: Ctrl-Shift-6, followed by x.

42. What two commands and what part of the command output tells you which suspended Telnet connection will be reconnected if you just press the Enter key, without any characters typed on the command line?

42. Answer: show sessions and where. Both commands supply the same information. The suspended session with an asterisk in the left side of the output line designates the session to which you would be connected if you just pressed Enter, with no characters on the command line.

43. Imagine that you typed a ping command and got five "!" back. What types of messages were sent through the network? Be as specific as possible.

43. Answer: ICMP echo request messages were sent by the ping command, with the remote host replying with five ICMP echo reply messages.

44. How do you make a router not ask for DNS resolution from a name server?

44. Answer: By using the no ip domain-lookup global configuration command.

45. Imagine that you are just logged in at the console of R1, and you Telnet to routers R2, R3, and R4 in succession, but you suspended your Telnet connection each time—in other words, all three Telnet connections go from R1 to the other three routers, respectively. What options do you have for reconnecting to R2?

45. Answer: You can use the resume 1 command or just use the 1 command.

46. Imagine that you are just logged in at the console of R1, and you Telnet to routers R2, R3, and R4 in succession, but you suspended your Telnet connection each time—in other words, all three Telnet connections go from R1 to the other three routers, respectively. What options do you have for reconnecting to R4?

46. Answer: You can use the resume 3 command or just use the 3 command. Or, you can simply press Enter, which resumes your connection to the last suspended Telnet connection—in this case, R4.

47. Which interior IP routing protocols support VLSM?

47. Answer: RIP-2, EIGRP, OSPF, and Integrated IS-IS.

48. Which IP routing protocols use distance vector logic?

48. Answer: RIP-1, RIP-2, and IGRP.

49. Which interior IP routing protocols are considered to converge quickly?

49. Answer: EIGRP, OSPF, and Integrated IS-IS.

5. What are the primary purposes of Flash memory in a Cisco router?

5. Answer: To store Cisco IOS and microcode files. If microcode is upgraded, the files also reside in Flash memory.

50. Compare distance vector and link-state protocols in terms of what information is sent in routing updates.

50. Answer: Distance vector protocols send sparse information, typically describing a subnet and a metric for each route. Link-state protocols send much more detailed topology information, describing each router and each link so that every router has a full conceptual picture of the network.

51. Explain the basic concept behind why some routing protocols support VLSM and some do not.

51. Answer: VLSM implies that different subnet masks are used for different subnets of the same network. To advertise subnets that have different sizes, the routing protocol must include the subnet mask information for each subnet in the routing updates. Routing protocols that do not support VLSM do not include the mask in the routing updates.

52. Explain the difference between interior and exterior routing protocols.

52. Answer: Interior routing protocols are designed to advertise detailed routing information about each subnet, typically inside a single company or organization. Exterior routing protocols are designed to advertise information about how to reach different organizations' networks through the Internet, and they purposefully try to reduce the detailed routing information to reduce the number of routes in Internet routers' routing tables.

53. Compare and contrast the types of information sent in distance vector routing updates versus link-state routing updates.

53. Answer: Distance vector routing updates contain a subnet number and a metric for each route. Link-state updates define much more detailed information, such as the identity of each router and which subnets each router is connected to. Therefore, the information in link-state updates is much more detailed than the equivalent information with distance vector updates. The more detailed information allows a linkstate protocol to build a mathematical representation of the network topology, whereas distance vector protocols simply know that subnets exist and where to send packets to reach those subnets.

54. What term describes the underlying logic behind the OSPF routing protocol?

54. Answer: Link state.

6. What is the main purpose of ROM in a Cisco router?

6. Answer: To store a small, limited-function version of IOS and to store bootstrap code. Typically, this type of IOS is used only during maintenance or emergencies.

7. What configuration command would be needed to cause a router to use a Cisco IOS image named c2500-j-l.112-14.bin on TFTP server 128.1.1.1 when the router is reloaded? If you forgot the first parameter of this command, what steps must you take to learn the correct parameters and add the command to the configuration? (Assume that you are not logged in to the router when you start.)

7. Answer: boot system tftp c2500-j-l.112-14.bin 128.1.1.1 As for the second part of the question: Log in from con/aux/telnet, type the enable command, type the enable password, type the configure terminal command, and type boot ?. Help appears for the first parameter of the boot command.

8. What two methods could a router administrator use to cause a router to load IOS stored in ROM?

8. Answer: Set the configuration register boot field to binary 0001, or add boot system rom to the configuration file and copy it to the startup configuration file. To set the configuration register to hex 2101, which would yield binary 0001 in the boot field, the config-register 0x2101 global configuration command would be used. A third method is to remove the Flash memory in a router and then reload the router.

9. What is the process used to update the contents of Flash memory so that a new IOS image in a file called c4500-d-mz.120-5.bin on TFTP server 128.1.1.1 is copied into Flash memory?

9. Answer: copy tftp flash. The other details—namely, the IP address of the TFTP server and the filename—are requested through prompts to the user.