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23.Router R1 has two BRI interfaces. Configure a dialer profile such that R1 can dial any of six different remote routers using any of the B channels on either BRI. Assume that all traffic is interesting. You may ignore the static route commands needed to send the packets out the correct interface. Do not use any SPIDs, and do not use CHAP. For other parameters not listed, you can make up values.
Answer:
dialer-list 2 protocol ip permit
!
interface dialer 0 encapsulation ppp
dialer map ip 172.16.2.1 broadcast 15551111111 dialer map ip 172.16.2.2 broadcast 15552222222 dialer map ip 172.16.2.3 broadcast 15553333333 dialer map ip 172.16.2.4 broadcast 15554444444 dialer map ip 172.16.2.5 broadcast 15555555555 dialer map ip 172.16.2.6 broadcast 1555666666 dialer-group 2
dialer pool 3
!
interface bri0 encapsulation ppp dialer pool-member 3
!
interface bri1 encapsulation ppp dialer pool-member 3
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“Do I Know This Already?” Quiz
1.Which of the following defines a protocol used between the Frame Relay DTE and the Frame Relay switch in the service provider’s network?
Answer: C. The LMI manages the link between the DTE and the switch, including notices when a VC comes up or goes down.
2.Which of the following defines a protocol or feature that matters to what the provider might do inside its network but that is transparent to the DTE/router using the Frame Relay service?
Answer: F. FRF.5 service interworking defines how a provider can use an ATM network between the Frame Relay switches. The routers outside the Frame Relay cloud cannot tell, and do not care, if FRF.5 is used or not.
3.What does DLCI stand for?
Answer: A
542Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
4.Imagine two Cisco routers, R1 and R2, using a Frame Relay service. R1 connects to a switch that uses LMI type ANSI T1.617, and R2 connects to a switch that uses ITU Q.933a. What can R1 and R2 configure for the LMIs to work correctly?
Answer: C, F. The correct keywords are in answer C. However, the routers autodetect the LMI type by default, so not bothering to configure the LMI also works.
5.FredCo has five sites, with routers connected to the same Frame Relay network. Virtual circuits (VCs) have been defined between each pair of routers. What is the fewest subnets that FredCo could use on the Frame Relay network?
Answer: A. A single subnet can be used in any Frame Relay topology, but with a full mesh, a single subnet can be used with no tricky issues related to routing protocols.
6.BarneyCo has five sites, with routers connected to the same Frame Relay network. VCs have been defined between each pair of routers. Barney, the president of the company, will fire anyone who configures Frame Relay without using point-to-point subinterfaces. What is the fewest subnets that BarneyCo could use on the Frame Relay network?
Answer: D. BarneyCo has a total of ten VCs. With all of them configured on point-to- point subinterfaces, you need ten subnets, because you need one subnet per VC.
7.BettyCo has five sites, with routers connected to the same Frame Relay network. VCs have been defined between each pair of routers. Betty, the president of the company, will fire anyone who configures anything that could just as easily be left as a default. Which of the following configuration commands, configured for the Frame Relay network, would get the engineer fired?
Answer: C, D, E. The lmi-type defaults to autodetect, which works fine. inverse-arp is on by default on the physical interface, so there is no need to turn it on (answer E), and there is no need for static maps (answer D).
8.WilmaCo has some routers connected to a Frame Relay network. R1 is a router at a remote site, with a single VC back to WilmaCo’s headquarters. The R1 configuration currently looks like this:
interface serial 0/0
ip address 10.1.1.1 255.255.255.0 encapsulation frame-relay
Wilma, the president, has heard that point-to-point subinterfaces are cool, and she wants you to change the configuration to use a point-to-point subinterface. Which of the following commands do you need to use to migrate the configuration?
Answer: A, E. The IP address moves to the subinterface, so it needs to be removed from the serial interface first (answer A). The encapsulation stays on the physical interface. The frame-relay interface-dlci command must be used on the subinterface so that the router knows which DLCI goes with which subinterface—even if only one DLCI exists.
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9.WilmaCo has another network, with a main site router that has ten VCs connecting to the ten remote sites. Wilma now thinks that multipoint subinterfaces are even cooler than point-to-point. The current main site router’s configuration looks like this:
interface serial 0/0
ip address 172.16.1.1 255.255.255.0 encapsulation frame-relay
Wilma wants you to change the configuration to use a multipoint subinterface. Which of the following do you need to use to migrate the configuration? (Note: DLCIs 101 through 110 are used for the ten VCs.)
Answer: F. You can code DLCI only on a frame-relay interface-dlci command, and you need one for each VC under the multipoint interface.
10.Which of the following commands lists the information learned by Inverse ARP?
Answer: F
Q&A
1.What two WAN data-link protocols define a method of announcing the interface’s Layer 3 addresses to other devices attached to the WAN?
Answer: PPP and Frame Relay. PPP uses control protocols specific to each Layer 3 protocol supported. Frame Relay uses Inverse ARP.
2.Explain the purpose of Inverse ARP, as well as how it uses Frame Relay broadcasts.
Answer: A router discovers the Layer 3 address(es) of a router on the other end of a VC when that other router sends an Inverse ARP message. The message is not a broadcast.
3.Would a Frame Relay switch connected to a router behave differently if the IETF option were deleted from the encapsulation frame-relay ietf command on that attached router?
Would a router on the other end of the VC behave any differently if the same change were made?
Answer: The switch does not behave differently. The other router, however, must also use IETF encapsulation. Otherwise, the routers will not look at the correct fields to learn the packet type.
4.What does NBMA stand for? Does it apply to X.25 networks or Frame Relay networks?
Answer: NBMA stands for nonbroadcast multiaccess. X.25 and Frame Relay are NBMA networks. Multiaccess really means that more than two devices are connected to the data link, because many other devices may be reached by a single device. For instance, Router1 might have a PVC to Router2 and Router3, making it multiaccess.
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10.What show command tells you when a PVC became active? How does the router know what time the PVC became active?
Answer: The show frame-relay pvc command lists the time since the PVC came up. You can subtract this time from the current time to derive the time at which the VC came up. The router learns about when PVCs come up and go down from LMI messages.
11.What show command lists Frame Relay information about mapping? In what instances does the information displayed include the Layer 3 addresses of other routers?
Answer: show frame-relay map lists Frame Relay information about mapping. The mapping information includes Layer 3 addresses when multipoint subinterfaces are used or when no subinterfaces are used. The two cases in which the neighboring routers’ Layer 3 addresses are shown are the two cases in which Frame Relay acts like a multiaccess network. With point-to-point subinterfaces, the logic works like a point-to-point link, in which the next router’s Layer 3 address is unimportant to the routing process.
12.True or false: The no keepalive command on a Frame Relay serial interface causes no further Cisco-proprietary keepalive messages to be sent to the Frame Relay switch.
Answer: False. This command stops LMI status inquiry messages from being sent. They are defined in Frame Relay Forum standards. Cisco sends proprietary keepalive messages on point-to-point serial and LAN interfaces.
13.What debug option shows Inverse ARP messages?
Answer: debug frame-relay events shows Inverse ARP messages, as shown in Example 11-18.
14.True or false: The Frame Relay map configuration command allows more than one Layer 3 protocol address mapping on the same configuration command.
Answer: False. The syntax allows only a single network layer protocol and address to be configured.
15.What is the name of the field that identifies, or addresses, a Frame Relay virtual circuit?
Answer: The data-link connection identifier (DLCI) is used to identify a VC. The number may be different on either side of the VC.
16.Describe the difference between FRF.5 and FRF.8 service interworking.
Answer: FRF.5 defines how Frame Relay switches can use an ATM VC between each other inside a service provider’s network. FRF.8 defines how two DTEs, like routers, can communicate with one router using a Frame Relay VC and the other router using an ATM VC. With FRF.5, the endpoint DTEs both connect to a Frame Relay cloud. With FRF.8, one DTE is directly connected to an ATM network.