CCNA 3 Study Guide Answers-All

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CED255 INTERNETWORKING III, Ver. 3.0 MODULE 1 STUDY GUIDE 1. VLSM stands for: Variable Length Subnet Masking 2. The purpose VLSM was designed was to: Conserve IP addresses when using IPv4, maximizing the number of useable IP addresses available in a network. 3. Dynamic routing protocols that support VLSM include:

OSPF RIP v. 2 EIGRP IS-IS

4. VLSM is sometimes referred to as __________________________. Subnetting a subnet 5. What is an autonomous system? its a network used in one enterprise 6. What is meant by route aggregation? Its the same thing as route summarization; its a way to summarize more than 1 route with one route entry in a routers routing table. 7. How can you take advantage of route aggregation using VLSM? Make sure your VLSM routes are not distant from each other. This way routes can be aggregated into one route in the routing table. 8. Is RIP v. 1 a classful or classless protocol? RIP v. 2? What does this mean? RIP v. 1 is classful; RIP v. 2 is classless. It means that RIP v. 1 must stay within the normal IP class system (A, B, C); classless means RIP v. 2 can use VLSM.

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9. Because its a new protocol, can RIP v. 2 hop more than 16 times? No. It still has many of the same characteristics of RIP v. 1, including hop count max of 16. 10. What is the configuration command to start RIP v. 2 running? Router(config)#router rip Router(config-router)#version 2 Router (config-router)#network [attached network(s)] or Router(config)#router rip ver 2 11. What show commands can you use to ensure that RIP v. 2 has started running? show ip route show ip protocols 12. What is the command to flush (clear) the routing table to force an update? clear ip route *13.

What will you see if you use the debug ip rip command? Youll see all the RIP updates as they are sent and received by the router.

14. How do you turn off the debugging function? Add no to the same command you used to turn it on, or you can use the no debug all to turn off all debugging. 15. What are the three (3) ways that routers learn about routes (networks)? Briefly explain each one.

Static routing - additions to the routing table put in by the network administrator Dynamic routing additions to the routing table are added automatically as routers update themselves using protocols Default routes - default routes added to the routing table by the network administrator that indicates the path to take when there is no known route to the destination

16. What are the two commands that can be used to enter a default route? ip route [network no.] [subnet mask] [next hop IP address] ip default-network [network to use as default]

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17.

If you want to use the ip route command to specify a default network, how would you enter it? ip route 0.0.0.0 0.0.0.0 [next hop IP address]

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CED255 INTERNETWORKING III, Ver. 3.0 MODULE 2 STUDY GUIDE 1. What are the two main ways of classifying dynamic IGP routing protocols? Distance-vector Link-state 2. Which ones are also known as shortest path first protocols? Why? Link-state, because thats exactly what they do: they look for the shortest path to the destination (regardless of whether or not this is the BEST path). 3. Fill in the following table: Type Distance-Vector Protocol(s) Advantages RIP v. 1 Easy to configure RIP v. 2 Updates regularly IGRP Disadvantages Takes up significant bandwidth Slow to converge Subject to routing loops RIP only has 1 metric Only RIP v. 2 can use VLSM Difficult to configure correctly Requires more memory and more powerful CPUs in routers Takes a lot of bandwidth when first started

Link-State

OSPF, IS-IS

Only sends out updates as needed Doesnt use much bandwidth to maintain tables Fast to converge Not subject to routing loops Knows complete topology of network so knows all routes Uses cost metrics instead of distance metrics Can use VLSM and CIDR

4. What are hello packets used for? Link state protocols use them to make sure a link is still active. Theyre very small packets.

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5. What are the five things that link state protocols use to maintain their tables:

Link-state advertisements (LSAs) A topological database The shortest path first (SPF) algorithm The resulting SPF tree A routing table of paths and ports to each network to determine the best paths for packets

6. What is meant by a link when talking about link state protocols? A link is an interface on a router. 7. How does a link state protocol build its topological database? What kind of information is in it? Its a database of the entire topology of the network and includes each link and how to reach it. Its built by using the LSAs that have been received by the router. 8. What are the three types of networks recognized by OSPF?

Broadcast multi-access, such as Ethernet Nonbroadcast multi-access (NBMA), such as frame relay Point-to-point networks

9. What is the DR and the BDR in an OSPF network? What do they do? DR Designated Router acts as the brains of the network. This router makes sure that all the LSAs are sent to all the other routers in the Area. BDR Backup Designated Router acts as the second in command of the network. If the DR goes down, the BDR takes over as DR. However, until the DR goes down, the BDR does not send out LSAs to any other routers in the Area. 10. What is the multicast address used by the DR to send out LSAs to all other OSPF routers? For LSAs just to other designated (and backup) routers? 224.0.0.5 224.0.0.6

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11. What is a hello packet used for and what is the multicast address used for it? Why this address? A hello packet is the way OSPF routers make sure that a link is still active. 224.0.0.5 is the address used so that all links are checked, not just DRs and BDRs. 12. What is the default hello interval? 10 seconds 13. What is the main area of an OSPF network designated? Area 0 (zero) 14. What is the command used to start OSPF routing on a router? Router(config)#router ospf [process ID] 15. What is the processor ID? Its an identifier for the OSPF routing process on the router 16. What is the command used to identify networks on an OSPF router? Router(config-router)#network [network no.] [wildcard-mask] area [area-id] 17. What does an OSPF router use for its router ID? How can you force a different ID? It uses the highest active IP address on any of the interfaces (where OSPF is running). If you want to change the ID, set a Loopback interface with a higher IP address so it will be forced to be the ID. 18. What is a Loopback? It is a logical (virtual) interface; not a physical (real) one 19. What is the command used to set a Loopback? Router(config)int Loopback[No.] Router(config-if) ip addr [IP number] [subnet mask] 20. What is the recommended subnet mask to use on a Loopback interface? 255.255.255.255

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21. What is a priority number used for? How can you change it? If there is more than 1 router in the Area (broadcast multi-access), then there must be a DR and maybe a BDR for the Area. The priority number is used to determine which routers will be the DR and the BDR. You can change it with the following command: Router(config-if)#ip ospf [priority number] 22. Why must you set the bandwidth on an interface running OSPF? What is the default bandwidth? How can you change it? Cost (the default metric of OSPF) uses bandwidth to determine the best route. The default is 1.544 Mbps. You can change it with the command: Router(config)#interface [type] [number] Router(config-if)#bandwidth [Kbps] 23. How can OSPF routers authenticate each other? They exchange passwords that only other OSPF routers will know. Use the commands below to set authentication: Router(config-if)#ip ospf authentication-key [password] Router(config-router)#area [area-number] authentication 24. What is the difference between default authentication and a message-digest authentication procedure? What is the command to do the latter? The default authentication sends the password in plain text; messagedigest encrypts the password thats sent. Router(config-if)#ip ospf authentication message-digest-key [key-id] md5 [encryption-type key] Router(config-router)#area [area-id] authentication message-digest 25. What is the relationship between the hello packet interval and the dead interval? What is meant by the dead interval? The dead interval is 4 times the hello interval (e.g., hello is 10 seconds, so dead is 40 seconds). The dead interval is the time used to determine that a link is down, or dead. In other words, if a hello isnt received from a link for the space of 4 times the normal hello interval, its considered dead.

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26. What would happen if routers in the OSPF network have different hello intervals configured? They wouldnt be able to talk to each other, so would be considered inactive. Hello intervals must be the same on all the routers in the network. 27. What is the best way to define a default route on an OSPF router? Use the quad zero command: Router(config)#ip route 0.0.0.0 0.0.0.0 [interface or IP address of next hop] 28. How can you make sure that this information is propagated to other routers in the area? Router(config-router)#default-information originate 29. List some of the show commands you can use to make sure that OSPF is functioning correctly. Show ip route Show ip protocol Show ip ospf interface Show ip ospf Show ip ospf neighbor [detail] (shows neighbor database) Show ip ospf database (shows topological database)

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CED255 INTERNETWORKING III, Ver. 3.0 MODULE 3 STUDY GUIDE 1. What is the difference between IGRP and EIGRP? EIGRP supports VLSM and CIDR, it has faster convergence times, its more scalable, and handles routing loops better than IGRP does. 2. Why is EIGRP referred to as a hybrid protocol? Because it primarily is a distance vector protocol, but also has some characteristics of a link state protocol, such as sending hello packets, sending only partial updates (instead of entire table exchanges), and using neighbor discovery. However, its easier to configure than OSPF. 3. Can IGRP and EIGRP routers talk to each other? Why or why not? Yes, they can talk because EIGRP is simply an enhancement of IGRP; theyre still basically the same protocol. 4. List the three tables that EIGRP maintains and briefly describe each one. Topology lists all routing tables in the AS; all learned routes are in this table Neighbor lists adjacent routers, their addresses and interfaces Routing lists best route to each destination network 5. List the five pieces of information found in the topology table and briefly describe each one.

Feasible distance The feasible distance (FD) is the lowest calculated metric to each destination. Route source The source of the route is the identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. Reported distance The reported distance (RD) of the path is the distance reported by an adjacent neighbor to a specific destination. Interface information The interface though which the destination is reachable Route status Routes are identified as being either passive (P), which means that the route is stable and ready for use, or active (A), which means that the route is in the process of being recomputed by DUAL.

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6. What is DUAL? The EIGRP distance vector algorithm is called the Diffusing Update Algorithm (DUAL). DUAL tracks all the routes advertised by neighbors. Composite metrics of each route are used to compare them. DUAL also guarantees that each path is loop free. DUAL inserts lowest cost paths into the routing table. These primary routes are known as successor routes. A copy of the successor routes is also placed in the topology table. 7. What is the difference between a feasible successor route and a :successor router? The successor route is the route in the routing table; in other words, the route considered the best to the destination. The feasible successor is the back-up route, or the next-best route. 8. What makes EIGRP able to support IP, IPX, and AppleTalk? It uses PDMs (protocol dependent modules). Also, for AppleTalk, it can actually act as the primarily protocol and AppleTalk doesnt even have to be running. 9. What is RTP? What does it do? Reliable Transport Protocol is a transport-layer protocol that can guarantee ordered delivery of EIGRP packets to all neighbors. This means EIGRP does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do. To stay independent of IP, EIGRP uses RTP as its own proprietary transport-layer protocol to guarantee delivery of routing information. What are the five EIGRP packet types? Briefly describe each one.

Hello - discover, verify, and rediscover neighbor routers Acknowledgement - to indicate receipt of any EIGRP packet during a reliable exchange Update - used when a router discovers a new neighbor. An EIGRP router sends unicast update packets to that new neighbor, so that it can add to its topology table Query - uses query packets whenever it needs specific information from one or all of its neighbors Reply - used to respond to a query

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What are the commands used to start EIGRP running on a router? router(config)# router eigrp [autonomous-system-number] router(config-router)# network [network number] router(config)#int [type] [number] router(config-if)# bandwidth [kilobits] router(config-if)# eigrp log-neighbor-changes If you do not want to summarize routes, what is the command to turn it off? Why might you not want to summarize (aggregate) routes? router(config-router)#no auto-summary If you have discontiguous subnets (subnets not right beside each other in the numbering scheme), route summarization probably wont work right. List some of the show commands you can use to verify that EIGRP is running correctly. show ip route show ip eigrp show ip protocol show ip eigrp neighbor [details] show ip eigrp interface show ip eigrp topology show ip eigrp traffic Which table built by EIGRP is considered the most important? List the fields of information contained in this table and briefly describe each one.

The neighbor table. Neighbor address network layer address of the neighbor router Hold time interval to wait without receiving anything from a neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello will reset the timer. Smooth Round-Trip Timer (SRTT) av...