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Cisco ICND2 Section 4 Wide Area Network
Cisco ICND2 Wide Area Network
Terms in this set (30)
1. Are DLCI addresses deﬁned by a Layer 2 or Layer 3 protocol?
1. Answer: DLCI addresses are deﬁned by a Layer 2 protocol. Frame Relay protocols do not deﬁne a logical addressing structure that can usefully exist outside a Frame Relay network; by deﬁnition, the addresses would be OSI Layer 2-equivalent.
2. What is the name of the ﬁeld that identiﬁes, or addresses, a Frame Relay virtual circuit?
2. Answer: The data-link connection identiﬁer (DLCI) is used to identify a VC.
3. True or False: "Frame Relay VCs provide a constant amount of bandwidth between two devices, typically routers—never more and never less." Defend your answer.
3. Answer: False. The provider assigns a guaranteed bandwidth, or CIR, for a VC, but the routers on either end of the VC can send more than the CIR of data. As long as the service provider has enough capacity to support it, the frames are forwarded over the VC.
4. Explain how many DS0 channels ﬁt into a T1, and why the total does not add up to the purported speed of a T1, which is 1.544 Mbps.
4. Answer: Each DS0 channel runs at 64 kbps. With 24 in a T1, the T1 speed seemingly would be 24 * 64 kbps, or 1.536 Mbps. T1 also includes 8 kbps for management, which, when added to the 1.536 Mbps total, gives you the full T1 rate—1.544 Mbps.
5. Deﬁne the term synchronous.
5. Answer: The imposition of time ordering on a bit stream. Practically, a device will try to use the same speed as another device on the other end of a serial link. 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.
6. Imagine a drawing with two routers, each connected to an external CSU/DSU. Each CSU/DSU is connected with a four-wire circuit. Describe the role of the devices in relation to clocking and synchronization.
6. Answer: The routers receive clocking from their respective CSU/DSUs. One of the two CSU/DSUs is conﬁgured as the master. The other CSU/DSU, as the slave, adjusts its clock to match the speed of the master CSU/DSU.
7. Imagine a drawing with two routers, each connected to an external CSU/DSU. Each CSU/DSU is connected with a four-wire circuit. List the words behind the acronyms DTE and DCE, and describe which devices in this imagined network are DTE and which are DCE.
7. Answer: DTE stands for data terminal equipment, and DCE stands for data communications equipment. The routers are DTEs, and the CSU/DSUs are DCEs.
8. Imagine a drawing with two routers, each connected to a Frame Relay switch over a local access link. Describe which devices in this imagined network are Frame Relay DTEs and which are Frame Relay DCEs.
8. Answer: The routers are DTEs, and the Frame Relay switches are DCEs.
9. Imagine a point-to-point leased line between two routers, with PPP in use. What are the names of the protocols inside PPP that would be used on this link? What are their main functions?
9. Answer: The PPP Link Control Protocol (LCP) controls and manages the link. The IP Control Protocol (IPCP) also would be used because you need a CP for each Layer 3 protocol. IPCP can assign IP addresses to devices on the other end of a link.
10. Deﬁne the terms DCE and DTE in the context of the physical layer and a point-to-point serial link.
10. 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.
11. 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.
11. 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 runningconﬁg lists the clockrate command under the interface, regardless of which of the two commands was entered in conﬁguration mode.
12. 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.
12. Answer: PPP uses an IP Control Protocol (IPCP) to announce, and sometimes assign, IP addresses to each end of the link.
13. Can PPP dynamically assign IP addresses? If so, is this feature always enabled?
13. 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.
14. Deﬁne the terms PAP and CHAP. Which one sends passwords in clear-text format?
14. 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.
15. CHAP conﬁguration uses names and passwords. Given Routers A and B, describe what names and passwords must match in the respective CHAP conﬁgurations.
15. 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.
16. Create a CHAP conﬁguration between two routers. Make up speciﬁc details as needed.
16. 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. The Fred router is configured as: Hostname Fred [ENTER], username Barney password Bedrock [ENTER], interface serial 0 [ENTER], encapsulation ppp [ENTER], ppp authentication chap. The Barney router is configured as:Hostname Barney username [ENTER], Fred password Bedrock [ENTER], interface serial 0 [ENTER], encapsulation ppp [ENTER], ppp authentication chap.
17. What two WAN data-link protocols deﬁne a method of announcing the interface's Layer 3 addresses to other devices attached to the WAN?
17. Answer: PPP and Frame Relay. PPP uses control protocols speciﬁc to each Layer 3 protocol supported. Frame Relay uses Inverse ARP.
18. Explain the purpose of Inverse ARP, as well as how it uses Frame Relay broadcasts.
18. 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.
19. 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?
19. 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.
20. What does NBMA stand for? Does it apply to X.25 networks or Frame Relay networks?
20. 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.
21. Which layer or layers of OSI are most closely related to the functions of Frame Relay? Why?
21. Answer: OSI Layers 1 and 2 are most closely related to the functions of Frame Relay. Frame Relay refers to well-known physical layer speciﬁcations. Frame Relay deﬁnes headers for delivery across the Frame Relay cloud, but it provides no addressing structure to allow VCs among many different Frame Relay networks. Thus, it is not considered to match OSI Layer 3 functions.
22. When Inverse ARP is used by default, what additional conﬁguration is needed to get IGRP routing updates to ﬂow over each VC, assuming IGRP has already been conﬁgured correctly?
22. Answer: No additional conﬁguration is required. The forwarding of broadcasts as unicasts can be enabled on each VC and protocol for which Inverse ARP is received.
23. Deﬁne the attributes of a partial-mesh and full-mesh Frame Relay network.
23. Answer: In a partial-mesh network, not all DTEs are directly connected with a VC. In a full-mesh network, all DTEs are directly connected with a VC.
24. What key pieces of information are required in the frame-relay map statement?
24. Answer: The pieces of information required are the Layer 3 protocol, the next-hop router's Layer 3 address, the DLCI to reach that router, and whether to forward broadcasts. Frame Relay maps are not required if Inverse ARP is in use.
25. Create a conﬁguration for Router1 that has Frame Relay VCs to Router2 and Router3 (DLCIs 202 and 203, respectively) on Router1's Serial1 interface. Use any IP addresses you like. Assume that the network is not fully meshed.
25. Answer: interface serial 1 [ENTER], encapsulation frame-relay [ENTER], interface serial 1.1 point-to-point [ENTER], ip address 184.108.40.206 255.255.255.0 [ENTER], frame-relay interface-dlci 202 [ENTER], interface serial 1.2 point-to-point [ENTER], ip address 220.127.116.11 255.255.255.0 [ENTER], frame-relay interface-dlci 203 [ENTER]. This is not the only valid configuration given the problem statement. However, because there is not a full mesh, point-to-point subinterfaces are the best choice. Cisco encapsulation is used by default. The LMI type is autosensed.
26. What show command tells you when a PVC became active? How does the router know what time the PVC became active?
26. 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.
27. What show command lists Frame Relay information about mapping? In what instances does the information displayed include the Layer 3 addresses of other routers?
27. 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 pointto-point link, in which the next router's Layer 3 address is unimportant to the routing process.
28. 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.
28. 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.
29. What debug option shows Inverse ARP messages?
29. Answer: debug frame-relay events shows Inverse ARP messages.
30. What is the name of the ﬁeld that identiﬁes, or addresses, a Frame Relay virtual circuit?
30. Answer: The data-link connection identiﬁer (DLCI) is used to identify a VC. The number may be different on either side of the VC.