Chapter 3's Review Questions

1: In the RIP scenario, why were you successful with using RIPv2 rather than RIPv1?
A1: Answer: RIPv2 is classless, and RIPv1 is classful. Classless routing protocols such as RIPv2, EIGRP, OSPF, and IS-IS support VLSM and summarization. All routing protocols support summarization, but the classful ones do that in a fixed manner at the class boundary.
2: A Cisco router maintains ARP entries much longer than most PCs. How can you remove all entries from the ARP cache on a Cisco router? It would be less detrimental to all to just remove an entry associated with a given interface. Can you do that on a router? On a Windows-based PC?
A2: Answer: On a router, clear arp does not truly clear the table; instead, it refreshes it. Unless in a test environment where it doesn't matter who you affect, you should use shut/no shut to remove the entries associated with a given interface. The command arp -d ip address on a Windows-based machine allows you to remove an entry at a time.
3: Draw a table comparing TCP/IP layers, protocols, applications, and utilities to the OSI model.
A3: Answer: See Table 3-2.
4: On a Cisco router, show ip route displays the routing table. What are the numbers in brackets []?
A4: Answer: The numbers in brackets are [administrative distance/composite metric].
5: Subnetting, aggregation, VLSM, CIDR, supernetting, and summarization are all about moving bit boundaries. Which one(s) move the network mask bit boundary to the right?
A5: Answer: Subnetting and VLSM.
6: Assume you moved into apartment 172.16.3.10 (host address) located at 172.16 Broad Creek Drive (network address). Other floors in the apartment building are numbered 172.16.1.0, 172.16.2.0, and 172.16.4.0. What floor (subnet) are you on? What are all the available hosts on that subnet? What is the directed broadcast address of your subnet?
A6: Answer: Because the other subnets are 172.16.1.0, 172.16.2.0, and 172.16.4.0, you must be on subnet 172.16.3.0. If you perform the binary math, you would in fact prove that you are on subnet (floor) 3. The hosts (other apartments on your floor) are as follows: 172.16.3.1 through 172.16.3.254. 172.16.3.255 is the directed broadcast address for your subnet. Again, if you work out the binary, you will see that the first available host is the subnet plus one, the directed broadcast is all 1s for the host bits, and the last available host is the broadcast address minus one.
7: Compare the protocol and port numbers for telnet, RIP, FTP, and TFTP.
A7: Answer: RIP (port 520) and TFTP (port 69) are both based on UDP, which is IP protocol number 17. Telnet (port 23) and FTP (ports 20, 21) are both based on TCP, which is protocol number 6. Refer to Figure 3-13 for more assistance.
8: You can ping by the IP address but not by the hostname. What is a very likely problem?
A8: Answer: Hostname resolution. Check your hosts files and DNS servers. You can test this now if you really want.
9: You need to forward DHCP requests to another subnet, but you do not want to forward NetBIOS communications. Is this possible?
A9: Answer: The command ip helper-address [DHCP_server_address] is required on your local router interface. By default, however, it allows TFTP, DNS, Time, two NetBIOS ports, two DHCP ports, and TACACS. You must specify which ports you want to forward and then the ports that you don't want to forward. To forward fewer than the eight default ports that IP helper opens up, you can use the ip forward-protocol udp [port] command for the ports you want to forward followed by the no ip forward-protocol udp [port] command for the ports you do not want to forward.
10: What IOS command enables you to verify that RIP sends broadcast routing updates? To what address are broadcast updates sent?
A10: Answer: debug ip rip shows you that routing updates are sent to 255.255.255.255.
11: Using 192.168.5.0/24, address the network according to the following requirements: three LAN segments—one with 125 hosts, one with 50 hosts, and one with 25 hosts—and at least two and maybe more WAN segments.
A11: Answer: I recommended that you start with host requirements, then work on your LAN requirements, and then work on the WAN requirements for VLSM. Remember that when you are solving for host bits, they are 0 bits. You should draw this out like Figure A-1 to understand the scenario and to truly see the bits:

  • Start with the maximum number of hosts and solve for x. 2x >= 125 hosts is 7 host bits. Note the seven 0 host bits in Figure A-1 where I assign 192.168.5.128/25 to the 125-host subnet.

  • Use VLSM for subnet 0 to continue. Solve for x. 2x >= 50 hosts to arrive at 6 host bits. Note the six host 0 bits in Figure A-1 where I assign 192.168.64.0/26 to the 50-host subnet.

  • Use VLSM for subnet 0 to continue. Solve for x. 2x >= 25 hosts to arrive at 5 host bits. Note the five host 0 bits in Figure A-1 where I assign 192.168.32.0/27 to the 25-host subnet.

  • Now that you have calculated the host and LAN segement requirements, use VLSM subnet 0 out to a /30 mask to maximize the WAN links. The WAN links can use 192.168.5.0/30, 192.168.5.4/30, 192.168.5.8/30, and so on as illustrated in Figure A-1.


Figure A-1.


12: You are having a problem with three subnets connected via two Cisco routers. Each router can ping its own interfaces but can't get to the far side of the other router. So you decide to put in the appropriate default route statement, but things still are not operational. You are not running routing protocols because default routes serve this scenario well. Can you spot the issue?
A12: Answer: You should draw this out and label the subnets to give you a picture of the problem. Alternatively, you can assume the scenario to be hosta connected to r1, r1 connected to r3, and hostc connected to r3. Obviously, this could be any number of things, and I would like to stress once more to use a structured approach such as in Chapter 1 and to divide and conquer to help you spot the particular issue. In looking at your configurations, you found no IP classless, so in effect your default routes were not working. After you turned on IP classless, you could route.
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