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Cisco ICND2 - Section 3 - Routing Protocols
Cisco ICND2 - Section 3 - Routing Protocols
Terms in this set (43)
1. Create a minimal conﬁguration enabling IP on each interface on a 2600 series router (two serial, one Ethernet). The Network Information Center (NIC) assigns you network 192.168.1.0. Your boss says that you need, at most, 60 hosts per LAN subnet. You also have point-to-point links attached to the serial interfaces. When choosing the IP address values and subnet numbers, you decide to start with the lowest numerical values. Assume that point-to-point serial links will be attached to this router and that EIGRP is the routing protocol. (Avoid using the zero subnet.)
1. Answer: interface ethernet 0/0 [ENTER], ip address 192.168.1.65 255.255.255.192 [ENTER], interface serial 0/0 [ENTER], ip address 192.168.1.129 255.255.255.252 [ENTER], interface serial 0/1 [ENTER], ip address 192.168.1.133 255.255.255.252 [ENTER], router eigrp 1 [ENTER], network 192.168.1.0 [ENTER]. Several correct answers are possible. Be sure to use a mask of 255.255.255.252 on the serial links and 255.255.255.192 on the LAN interface. Also be sure to configure EIGRP just like the answer shown here, although you can use a number other than 1 for the autonomous system number. Also, this solution avoids using the zero subnet.
2. Write down the steps you would take to migrate from RIP to OSPF in a router whose current RIP conﬁguration includes only router rip followed by a network 10.0.0.0 command. Assume a single OSPF area, and use as few network commands as possible.
2. Answer: Issue the following commands in configuration mode: router ospf 5 [ENTER], network 10.0.0.0 0.255.255.255 area 0 [ENTER], no router rip.
3. Create a conﬁguration for EIGRP on a router with these interfaces and addresses: e0 using 10.1.1.1, e1 using 126.96.36.199, s0 using 10.1.2.1, and s1 using 188.8.131.52. Use AS 5.
3. Answer: router eigrp 5 [ENTER], network 10.0.0.0 [ENTER], network 184.108.40.206 [ENTER]. If you noticed that 220.127.116.11 is not a valid Class A, B, or C address, you get full credit. A new address is needed for Ethernet1, with a matching network command.
4. Create a conﬁguration for EIGRP on a router with these interfaces and addresses: e0 using 18.104.22.168, e1 using 22.214.171.124, s0 using 126.96.36.199, and s1 using 188.8.131.52.
4. Answer: router eigrp 1 [ENTER], network 184.108.40.206 [ENTER], network 220.127.116.11 [ENTER], network 18.104.22.168 [ENTER], network 22.214.171.124 [ENTER]. Because four different networks are used, four network commands are required. If you noticed that this question does not specify the AS number (1 in this example) but configures one, you get full credit. A few of these network numbers are used in examples; memorize the range of valid Class A, B, and C network numbers.
5. From a router's user mode, without using debugs or privileged mode, how can you determine what routers are sending you EIGRP routing updates?
5. Answer: The show ip protocol command output lists the routing sources—the IP addresses of routers sending updates to this router. Knowing how to determine a fact without looking at the conﬁguration will better prepare you for the exam. Also, the show ip route command lists next-hop router IP addresses. The next-hop routers listed identify the routers that are sending routing updates. The show ip eigrp neighbor and show ip eigrp interface commands also list neighbors that by deﬁnition send routing updates.
6. If the command router eigrp 1, followed by network 10.0.0.0, with no other network commands, is conﬁgured in a router that has an Ethernet0 interface with IP address 126.96.36.199, does EIGRP send updates out Ethernet0?
6. Answer: No. There must be a network statement for network 188.8.131.52 before EIGRP advertises out that interface. The network command simply selects the connected interfaces on which to send and receive updates.
7. If the command router ospf 1, followed by network 10.0.0.0 0.255.255.255 area 0, with no other network commands, is conﬁgured in a router that has an Ethernet0 interface with IP address 10.10.1.1, does OSPF send updates out Ethernet0?
7. Answer: Not necessarily. OSPF must discover other OSPF neighbors on the interface before it advertises routing information (LSAs.)
8. If the commands router eigrp 1 and network 10.0.0.0 are conﬁgured in a router that has an Ethernet0 interface with IP address 184.108.40.206, mask 255.255.255.0, does this router have a route to 220.127.116.11?
8. Answer: Yes. The route is in the routing table because it is a directly connected subnet, not because of any action by EIGRP.
9. Which command lists all IP routes learned via OSPF, and only those routes?
9. Answer: The show ip route ospf command lists only OSPF-learned routes.
10. Compare and contrast the type of information exchanged in routing updates sent by distance vector routing protocols versus link-state protocols.
10. Answer: Distance vector protocols advertise subnets and their associated metric values. Link-state protocols advertise information about routers and subnets, or links, in the network, along with metric information for the links. Link-state protocols describe the full topology in the network. As a result, link-state routing information is much more detailed than distance vector protocols.
11. Deﬁne balanced hybrid, and give an example of a balanced hybrid protocol.
11. Answer: Cisco uses the term balanced hybrid to describe a class of routing protocols that have some distance vector characteristics and some link-state characteristics. Currently, only EIGRP falls into this category.
12. Describe how balanced hybrid protocols differ from distance vector protocols in terms of how a router notices that a neighboring router has failed.
12. Answer: Distance vector routing protocols rely on regular full routing updates from each neighboring router. If a router fails to receive those updates for a period of time, the router that fails to receive the updates assumes that the other router has failed. Balanced hybrid protocols do not send full updates regularly; instead, they send periodic Hello messages. If Hello messages are not received for some timeout period, the router that is no longer receiving the updates assumes that the other router has failed.
13. List the distance vector loop-avoidance features used by OSPF, such as split horizon.
13. Answer: OSPF, as a link-state routing protocol, does not need to use any of the distance vector loop-avoidance features. Loop avoidance is effectively built into the routing protocol.
14. List two OSPF features that help decrease the size of the OSPF topology database.
14. Answer: If you use multiple OSPF areas, the size of the database in routers that are not ABRs decreases. Also, by using some stub area type, you can reduce the size of the topology database even further.
15. Assume that you must choose between OSPF and EIGRP for a routing protocol in a new network you are building. List and explain the most compelling reason to choose OSPF and the most compelling reason to choose EIGRP.
15. Answer: OSPF converges fast, like EIGRP, but it is an open standard, unlike EIGRP. Therefore, OSPF would more easily allow routers from multiple vendors to be used. EIGRP converges quickly, like OSPF, but requires little or no engineering design, whereas OSPF requires signiﬁcant engineering design effort in larger networks.
16. Name the two commands typically used to create a default route for a router.
16. Answer: The ip default-network and ip route 0.0.0.0 0.0.0.0 commands accomplish the goal of having the router use a known route as the default for packets that are not matched in the routing table. The ip route 0.0.0.0 0.0.0.0 command uses the fact that network 0.0.0.0 is used by Cisco IOS software to represent the default network.
17. Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and subnets are known, except that there is also a default route (0.0.0.0) in the routing table. A packet destined for 18.104.22.168 arrives at the router. What conﬁguration command determines whether the default route is used in this case?
17. Answer: The packet is routed using the default route, regardless of other conﬁguration commands. In this scenario, in which the Class A, B, or C network is known, there is no match for the destination in the known subnets, and a default exists, so the default must be used.
18. Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and its subnets are known, except that there is also a default route (0.0.0.0) in the routing table. A packet destined for 10.1.1.1 arrives at the router, but no known subnet of network 10 matches this destination address. What conﬁguration command determines whether the default route is used in this case?
18. Answer: If the command ip classless is conﬁgured, the packet is routed using the default route. If no ip classless is conﬁgured, the packet is discarded.
19. What feature supported by EIGRP allows it to support manual VLSM?
19. Answer: The association and transmission of mask information with each route allows VLSM support with any routing protocol.
20. List the interior IP routing protocols that have autosummarization enabled by default. Which of these protocols allow autosummarization to be disabled using a conﬁguration command?
20. Answer: RIP-1, IGRP, EIGRP, and RIP-2 all have autosummarization enabled by default. EIGRP and RIP-2 can disable this feature.
21. Which interior IP routing protocols support manual route summarization?
21. Answer: EIGRP, OSPF, IS-IS, and RIP-2 support route summarization. (Give yourself credit on this question if you included OSPF and EIGRP.)
22. Assume that several subnets of network 172.16.0.0 exist in a router's routing table. What must be true about these routes for the output of the show ip route command to list mask information only on the line that lists network 172.16.0.0 but that doesn't show mask information on each route for each subnet?
22. Answer: If all the subnets of 172.16.0.0 use the same mask, the output of the show ip route command lists only the mask in the heading line for the network. If VLSM were in use, each route for each subnet would reﬂect the mask used in that case.
23. Router A and Router B are connected via a point-to-point serial link. Router A's interfaces use IP address 172.16.1.1, mask 255.255.255.0 and address 172.16.2.1, mask 255.255.255.0. Router B's interfaces use address 172.16.2.2, mask 255.255.255.0 and address 10.1.1.1, mask 255.255.254.0. Is VLSM in use? Explain your answer.
23. Answer: Although two different masks are used, VLSM is not used. VLSM implies that two different masks are in use in the same Class A, B, or C network. In this example, only one mask is used for each classful network.
24. What is the smallest summarized route that summarizes the subnets 10.1.63.0, 10.1.64.0, 10.1.70.0, and 10.1.71.0, all with mask 255.255.255.0?
24. Answer: Only the ﬁrst 17 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.1.0.0, mask 255.255.128.0.
25. What is the smallest summarized route that summarizes the subnets 10.5.111.0, 10.5.112.0, 10.5.113.0, and 10.5.114.0, all with mask 255.255.255.0?
25. Answer: The ﬁrst 19 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.5.96.0, mask 255.255.224.0.
26. What is the smallest summarized route that summarizes the subnets 10.5.110.32, 10.5.110.48, and 10.5.110.64, all with mask 255.255.255.248?
26. Answer: The ﬁrst 21 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.5.108.0, mask 255.255.248.0.
27. Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which are classless?
27. Answer: EIGRP and OSPF
28. Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which support VLSM?
28. Answer: EIGRP and OSPF
29. Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which advertise mask information along with subnet numbers?
29. Answer: EIGRP and OSPF
30. Of the terms classful routing, classful routing protocol, classless routing, and classless routing protocol, which describe a feature that affects when a router uses the default route?
30. Answer: Classful routing and classless routing deﬁne rules by which a router chooses whether to use the default route.
31. What allows for the successful use of a discontiguous Class A, B, or C IP network— classful routing, classful routing protocol, classless routing, or classless routing protocol?
31. Answer: Whether a routing protocol is or is not classless deﬁnes whether it can support discontiguous networks. Only a classless routing protocol supports discontiguous networks.
32. Compare and contrast route summarization and autosummarization.
32. Answer: Route summarization allows an engineer to choose a summary route to conﬁgure, advertising the summary as opposed to the more speciﬁc routes. Autosummarization summarizes only a Class A, B, or C network number, and only at the boundary between that network and another network.
33. Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which use autosummarization by default and also cannot have autosummarization disabled?
33. Answer: RIP-1 and IGRP
34. What command switches a router from classless routing to classful routing?
34. Answer: The no ip classless global conﬁguration command disables classless routing, thereby enabling classful routing.
35. Deﬁne private addressing as deﬁned in RFC 1918.
35. Answer: Some hosts will never need to communicate with other hosts across the Internet. For such hosts, assigning IP addresses from registered networks wastes IP addresses. To conserve IP addresses, a set of network numbers, called private addresses, has been reserved and can be used in these cases to help conserve IP addresses for use over the Internet.
36. List the range of private networks deﬁned in RFC 1918.
36. Answer: Class A network 10.0.0.0, Class B networks 172.16.0.0 to 172.31.0.0, and Class C networks 192.168.0.0 to 192.168.255.0.
37. Does CIDR affect the size of Internet routing tables? If so, what does it do to those routing tables?
37. Answer: CIDR allows ISPs to summarize multiple Class A, B, or C networks, typically Class C networks, into summary routes. It shortens the length of the IP routing table, improving Internet routing performance.
38. Deﬁne NAT and explain the basics of its operation.
38. Answer: Network Address Translation is a mechanism for allowing hosts with private addresses or addresses that conﬂict with IP addresses from a registered network to communicate with hosts over the Internet. The basic operation involves the NAT router's changing the IP addresses in packets to and from these hosts so that only legitimately registered IP addresses are used in ﬂows through the Internet.
39. Describe the process used by IP routers to perform fragmentation and reassembly of packets.
39. Answer: When a packet must be forwarded, but the packet is larger than the maximum transmission unit (MTU) size for the outgoing interface, the router fragments the packet as long as the Don't Fragment bit is not set in the packet header. No IP router reassembles the fragments; fragments are reassembled at the ﬁnal destination host.
40. How many Class B-style networks are reserved by RFC 1918 private addressing?
40. Answer: Sixteen Class B networks are reserved for use as private networks in RFC 1918—networks 172.16.0.0 to 172.31.0.0.
41. Describe why ARP requests use an Ethernet broadcast address instead of an Ethernet unicast address.
41. Answer: Hosts use ARP when they need to learn the MAC address of another IP host. Because the host does not know the MAC address, it would be impossible to use an Ethernet unicast, because the host would not know the right MAC address to put in the destination address ﬁeld in the Ethernet frame.
42. 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.
42. Answer: interface fastethernet 0/0.1 [ENTER], ip address 10.1.1.1 255.255.255.0 [ENTER], encapsulation isl 1 [ENTER], interface fastethernet 0/0.2 [ENTER], ip address 10.1.2.1 255.255.255.0 [ENTER], encapsulation isl 2 [ENTER].
43. Describe how NAT overload manages to support more than one inside local IP address using a single inside global address.
43. 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.
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