536 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
Chapter 10
“Do I Know This Already?” Quiz
1.How fast do the D channels run on a BRI and PRI, respectively?
Answer: E
2.Which of the following protocols defines call setup signaling for ISDN?
Answer: E
3.Which of the following reference points are used by ISDN BRI interfaces in a Cisco router?
Answer: C, D. The router can use a serial interface with the R reference point, but the question asks about ISDN interfaces in the router.
4.Which of the following PRI function groups is appropriate for use with a PC with a simple serial interface?
Answer: E. BRIs support different reference points and function groups, but PRIs do not.
5.Which of the following encoding formats could be used on a PRI in North America?
Answer: A, B. In North America, PRIs are based on T1 circuits, which support either AMI or B8ZS encoding.
6.Assume that a router has an IP ACL numbered 109 that permits traffic going to IP address 10.1.1.1. Which of the following commands correctly refers to the ACL, causing traffic sent to IP address 10.1.1.1 to be considered “interesting”?
Answer: E. The dialer-list global command first refers to all IP traffic and then can optionally refer to an IP ACL to further refine the set of IP traffic that DDR considers “interesting.”
7.Which of the following could be necessary in a configuration that uses DDR and ISDN BRI interfaces?
Answer: A, D. ISDN configuration requires the switch type in most cases. SPIDs are needed only when the service provider’s switches expect to use the SPIDs as a basic authentication mechanism. Because there is only one option each for BRI encoding and framing, you never need to configure them.
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8.Which of the following could be necessary in a configuration that uses DDR and ISDN PRI interfaces in North America?
Answer: A, B, C, E. ISDN configuration requires the switch type in most cases. For PRI, you have two choices for encoding and framing. You also need to tell the router which DS0 channels will be used in the PRI.
9.When you configure DDR using the style of DDR configuration called dialer profiles, which of the following commands are useful on the dialer interface?
Answer: D. The encapsulation must be on the physical interfaces as well, but it is also needed on the dialer interface.
10.Which of the following interface subcommands enables multilink PPP?
Answer: A
Q&A
1.What does LAPD stand for? Is it used as the Layer 2 protocol on dialed ISDN bearer channels? If not, what is?
Answer: LAPD stands for Link Access Procedure on the D channel. LAPD is used not on bearer channels but on the signaling channel. PPP is typically used on bearer channels.
2.What do ISDN, BRI, and PRI stand for?
Answer: ISDN stands for Integrated Services Digital Network. BRI stands for Basic Rate Interface. PRI stands for Primary Rate Interface.
3.Define function group. List two examples of function groups.
Answer: A function group is a set of ISDN functions that a device implements. NT1, NT2, TE1, TE2, and TA are all function groups.
4.Define the term reference point. Give two examples of reference points.
Answer: A reference point is an interface between function groups. R, S, T, and U are reference points. S and T are combined in many cases and are then called the S/T reference point. Reference points refer to cabling, which implies the number of wires used. In particular, the S and T points use a four-wire interface, and the U interface uses a two-wire cable.
538Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
5.How many bearer channels are in a BRI? What about a PRI in North America? What about a PRI in Europe?
Answer: BRI uses two bearer channels and one signaling channel (2B+D). PRI uses 23B+D in North America and 30B+D in Europe. The signaling channel on BRI is a 16kbps channel; on PRI, it is a 64-kbps channel.
6.True or false: ISDN defines protocols that can be functionally equivalent to OSI Layers 1, 2, and 3. Explain your answer.
Answer: True. Reference points in part define the physical interfaces. Used on the signaling channel, LAPD is a data-link protocol. SPIDs define a logical addressing structure and are roughly equivalent to OSI Layer 3.
7.What reference points do ISDN BRI interfaces on Cisco routers use?
Answer: A BRI interface with an S/T reference point, or a BRI with a U reference point, can be bought from Cisco. With an S/T interface, an external NT1, NT2, or NT1/NT2 device is required. With the U interface, no external device is required.
8.Is LAPD used on ISDN channels? If so, which ones?
Answer: LAPD is used only on ISDN D channels to deliver signaling messages to the local ISDN switch. Many people don’t understand the function of LAPD, thinking it is used on the B channels after the dial is complete. The encapsulation chosen in the router configuration determines the data-link protocol on the bearer channels. Cisco routers do not have an option to turn off LAPD on the signaling channel.
9.What standards body defines ISDN protocols?
Answer: The International Telecommunications Union (ITU) defines ISDN protocols. This group was formerly called CCITT.
10.What ISDN functions do standards ITU-T Q.920 and Q.930 define? Does either standard correlate to an OSI layer?
Answer: Q.920 defines ISDN data-link specifications, such as LAPD; Q.930 defines Layer 3 functions, such as call setup messages. I.440 and I.450 are equivalent to Q.920 and Q.930, respectively.
11.What ISDN functions does standard ITU-T I.430 define? Does it correlate to an OSI layer?
Answer: I.430 defines ISDN BRI physical layer specifications. It is similar to OSI Layer
1.I.430 has no Q-series equivalent specification.
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12.What does SPID stand for? What does it mean?
Answer: SPID stands for service profile identifier. An optional feature, it is used for basic authentication between the telco switch and the device on the other end of the line, such as a router.
13.Define TE1, TE2, and TA. Which implies that one of the other two must be in use?
Answer: TE1 stands for Terminal Equipment 1. TE2 stands for Terminal Equipment 2. TA stands for terminal adapter. A TE2 device requires a TA. A TE2 uses the R reference point. An S reference point is needed to perform ISDN signaling. It is provided in that case by the TA.
14.How many B channels are there on a PRI in countries where the PRI is based on a T1? On an E1?
Answer: There are 23 B channels on a PRI in countries where the PRI is based on a T1. There are 30 B channels on a PRI in countries where the PRI is based on an E1.
15.What reference point is used between the customer premises and the phone company in North America? What about in Europe?
Answer: The U interface is used in North America. Elsewhere, the T interface is used. The NT1 function, the dividing point between the T and U reference points, is implemented in telco equipment outside North America.
16.What problem does multilink PPP solve when multiple B channels have circuits set up between a pair of routers?
Answer: MLP balances the traffic across multiple links efficiently, distributing the traffic evenly across the multiple links. Without MLP, the routers would let the routing process balance the traffic, which typically does not result in even balancing across the links.
17.What is the syntax of an interface command used to configure the encapsulation, IP address, and DDR parameters on a PRI in North America? What is the significance of entering a colon and a number after entering the interface number?
Answer: The interface serial 0/0:23 command places you in interface configuration mode for a PRI’s D channel. The D channel is where you place the interface configuration commands for DDR. The :23 identifies the last DS0 channel in the PRI. The numbering of the channels in the interface command starts at 0, so the 24th channel is number 23.
540Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
18.What data-link (OSI Layer 2) protocols are valid on an ISDN B channel?
Answer: HDLC, PPP, and LAPB are all valid options. PPP is the preferred choice, however. If you’re using DDR to more than one site, PAP or CHAP authentication is required. If it is used, PPP must also be used. PPP also provides automatic IP address assignment, which is convenient for PC dial-in.
19.Define MLPPP. Describe the typical home or small office use of MLPPP.
Answer: MLPPP stands for Multilink Point-to-Point Protocol. It is used to treat multiple B channels as a single link, because MLPPP fragments packets and sends different fragments across the multiple links to balance the traffic. MLPPP is very useful for sharing two B channels in a home or small office. so that you can increase and use the available bandwidth.
20.Configure ISDN interface BRI1, assuming that it is attached to a DMS-100 ISDN switch and that it uses only one SPID of 404555121201.
Answer:
isdn switch-type basic-dms100 interface bri1
isdn spid1 404555121201
The switch-type command is required. SPIDs are required with only some switches.
21.Describe the decision process performed by the Cisco IOS software to attempt to dial a connection using Legacy DDR.
Answer: First, some traffic must be routed out the interface to be dialed; this is typically accomplished by adding static routes pointing out the interface. Then, “interesting” traffic must be defined; any packets routed out the interface that are considered interesting cause the interface to be dialed.
22.If packets from 10.1.1.0/24 are “interesting” in relation to DDR configuration, such that packets from 10.1.1.0/24 cause a DDR connection out interface BRI0, list the configuration commands that make the Cisco IOS software think that those packets are interesting on BRI0. (If you have not yet studied access lists, you may simply note what kind of packets you would match with an access list, rather than writing down the full syntax.)
Answer: The following access list defines the packets from 10.1.1.0/24. The dialer-list command defines the use of access-list 1 to decide what is interesting. The dialer-group command enables that logic on interface BRI0.
access-list 1 permit 10.1.1.0 0.0.0.255
!
dialer-list 2 protocol ip list 1
!
interface bri 0 dialer-group 2