532Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

14.Imagine that host 10.1.1.1 in Figure 8-21 sends a packet to the server at 170.1.1.1 and that the NAT router also happens to fragment the packet. Inside the context of this network, explain how reassembly of the fragments into the original packet occurs.

Answer: The endpoint host for which the packets are destined always reassembles the fragments. In this case, host 170.1.1.1 reassembles the packet.

15.Imagine that R1 has an interface, FastEthernet 0/0, that uses ISL trunking to a switch. R1 needs to route between VLAN 1 and VLAN 2. Create a valid router configuration.

Answer:

interface fastethernet 0/0.1

ip address 10.1.1.1 255.255.255.0 encapsulation isl 1

!

interface fastethernet 0/0.2

ip address 10.1.2.1 255.255.255.0 encapsulation isl 2

16.Describe how NAT overload manages to support more than one inside local IP address using a single inside global address.

Answer: NAT overload uses Port Address Translation (PAT) to essentially translate both the source address and port numbers for incoming inside packets. By allocating unique port numbers to be used with a single inside global IP address, NAT overload can make multiple sockets on multiple inside hosts appear to be multiple sockets, but from a single host, on the public part of the network.

Chapter 9

“Do I Know This Already?” Quiz

1.Which of the following types of WAN connections allow the direct use of PPP as the encapsulation type on the interface?

Answer: A, C

2.Which of the following commands are required on the router connected to a DTE cable to make the serial link between two routers work when the two routers are connected in a lab using a DTE and DCE cable and no CSU/DSUs?

Answer: E. None of these commands is needed at the DTE. The clock rate 56000 command, or one like it, is needed on the router with a DCE cable.

3.Which of the following commands make a serial interface revert to the default data-link protocol if it is currently configured with PPP?

Answer: E, G. HDLC is the default setting.

Chapter 9 533

4.Which of the following authentication protocols authenticates a device on the other end of a link without sending any password information in clear text?

Answer: C. Of these answers, only PAP and CHAP are authentication protocols. PAP sends the password in clear text.

5.Imagine that two routers, R1 and R2, have a leased line between them. Each router had its configuration erased and was then reloaded. R1 was then configured with the following commands:

interface s0/0 encapsulation ppp

ppp authentication chap

Which of the following configuration commands can complete the configuration on R1 so that CHAP can work correctly? Assume that R2 has been configured correctly and that the password is fred.

Answer: D. R1 needs to know the password associated with the router on the other end of the link—namely, R2.

6.Which of the following is a reason to avoid using link compression?

Answer: C. “Reducing” available CPU cycles is an odd way to say that compression uses CPU in the router, which is one reason to choose not to perform compression.

7.Which of the following does PPP use as part of how it notices when a link has been looped, causing packets sent by a router to be returned to that router?

Answer: B

8.Which of the following are protocols defined as part of PPP?

Answer: B, D

Q&A

1.Define the terms DCE and DTE in the context of the physical layer and a point-to-point serial link.

Answer: At the physical layer, DTE refers to the device that looks for clocking from the device on the other end of the cable on a link. The DCE supplies that clocking. For example, the computer is typically the DTE, and the modem or CSU/DSU is the DCE. At the data link layer, both X.25 and Frame Relay define a logical DTE and DCE. In this case, the customer premises equipment (CPE), such as a router and CSU/DSU, is the logical DTE, and the service provider equipment (the Frame Relay switch and CSU/ DSU) is the DCE.

534Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

2.Identify the command used to set the clock rate on an interface in which a DCE cable has been inserted. Compare and contrast the two styles of the command that may be entered in configuration mode versus which style shows up in the configuration.

Answer: The clock rate command and the clockrate command both set the rate at which an interface clocks over a serial DCE cable. When you ask for help while in interface configuration mode, the clock rate command is shown, but help does not list the clockrate command. However, after you issue the command, a show running-config lists the clockrate command under the interface, regardless of which of the two commands was entered in configuration mode.

3.Name one WAN data-link protocol used on point-to-point leased lines that defines a method of announcing the interface’s IP addresses to other devices attached to the WAN.

Answer: PPP uses an IP Control Protocol (IPCP) to announce, and sometimes assign, IP addresses to each end of the link.

4.Can PPP dynamically assign IP addresses? If so, is this feature always enabled?

Answer: PPP’s IPCP protocol can assign an IP address to the device on the other end of the link. This process is not required and is not performed by default. PPP usually does address assignment for dial access, such as when a user dials an Internet service provider.

5.Create a configuration to enable PPP on serial 0 for IP. Make up IP Layer 3 addresses as needed.

interface serial 0

ip addr 1.1.1.1 255.255.255.0 encapsulation ppp

Answer: encapsulation ppp is all that is needed for PPP.

6.Define the terms PAP and CHAP. Which one sends passwords in clear-text format?

Answer: PAP stands for Password Authentication Protocol. CHAP stands for Challenge Handshake Authentication Protocol. PAP sends passwords as simple text, whereas CHAP uses MD5 hashing to protect the password contents.

7.CHAP configuration uses names and passwords. Given Routers A and B, describe what names and passwords must match in the respective CHAP configurations.

Answer: Router A has name B and a corresponding password configured. Router B has name A and the same password configured. The names used are the host names of the routers unless the CHAP name is configured.

Chapter 9 535

8.What field has Cisco added to the HDLC header, making it proprietary?

Answer: A Protocol Type field has been added to allow support for multiprotocol traffic. HDLC was not originally designed to allow for multiprotocol support.

9.List the types of packet switching mentioned in this chapter. Identify the one for which all traffic is segmented into smaller pieces.

Answer: Frame Relay, X.25, and ATM were all mentioned as packet switching services. ATM, also called cell switching, segments all traffic into 53-byte cells before transmission and reassembles the cells into the original frame after they are through the WAN.

10.Two-wire leased circuits are seldom used today, and certainly not between two routers. What problem was solved with the advent of four-wire leased circuits versus two-wire?

Answer: Two-wire leased lines could be used as each single pair, for transmission in only one direction at a time. With a four-wire circuit, each pair could be used for transmission in a different direction, allowing full-duplex data transmission.

11.Define synchronous in relation to leased lines.

Answer: The imposition of time ordering on a bit stream. Practically speaking, a device tries to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.

12.Create a CHAP configuration between two routers. Make up specific details as needed.

Answer: Your configuration will probably be a little different from the one shown here. To be correct, you need the username global commands that refer to the other router’s host name, with correct case and the same password on each side. PPP must be enabled on each serial link, and ppp authentication chap must be on each interface.