| The answers provided in this section are not necessarily the only possible answers to the questions. The questions are designed to test your knowledge and to give practical exercise in certain key areas. This section is intended to test and exercise skills and concepts detailed in the body of this chapter. If your answer is different, ask yourself whether it follows the tenets explained in the answers provided. Your answer is correct not if it matches the solution provided in the book, but rather if it has included the principles of design laid out in the chapter. In this way, the testing provided in these scenarios is deeper: It examines not only your knowledge, but also your understanding and ability to apply that knowledge to problems. If you do not get the correct answer, refer back to the text and review the subject tested . Be certain to also review your notes on the question to ensure that you understand the principles of the subject. Scenario 15-1 Answers | 1: | Using the diagram in Figure 15-11 as a reference, issue the commands that need to be configured at each country or autonomous system. The private autonomous system numbers range from 64,512 to 65,535. Figure 15-11. Diagram for Scenario 15-1  Figure 15-11 has been simplified and does not contain 250 autonomous systems, as the case study suggests. | | A1: | The commands configured at each country or autonomous system would be the same structurally, although the details, such as the IP addresses and the autonomous system numbers, would change. Example 15-4 shows the configuration of the autonomous system 64522, which is the San Francisco router. It has been assigned network 10.2.0.0. Example 15-4. Scenario 15-1 Configuration Router(config)# router bgp 64522 no auto-summary Neighbor 10.1.100.1 remote-as 64521 Neighbor 10.3.100.1 remote-as 64523 Neighbor 10.4.100.1 remote-as 64524 Neighbor 10.5.100.1 remote-as 64525 Neighbor 10.6.100.1 remote-as 64526 Neighbor 10.7.100.1 remote-as 64527 Neighbor 10.8.100.1 remote-as 64528 Neighbor 10.9.100.1 remote-as 64529 Neighbor 10.10.100.1 remote-as 64530 !neighbor commands omitted for brevity Neighbor 10.250.100.1 remote-as 64750 network 10.2.0.0 The protocol has had the neighbors in each autonomous system defined with their next -hop IP address and the number of the autonomous system to which they are connecting. The no auto-summary command is used to ensure that the subnets of network 10.0.0.0 are advertised; otherwise , each subnet would need to be defined as a network command. | | 2: | The BGP network is a full-mesh network. Are there going to be any scaling problems ensuing from this? | | A2: | There should not be a problem with this design. Although there is an enormous number of TCP connections, the traffic is minimal, particularly if aggregation is configured. Also, BGP sends only triggered updates, so if the network is stable and route aggregation is configured, bandwidth should not be a concern. However, considering the propagation delays, the BGP timers might need to be reviewed. Regarding CPU, a high-power router should be used for this purpose. Each time a new BGP neighbor is added, there will be an increasing number of eBGP peer configurations for the network administrator. | | 3: | What commands would indicate that there was a problem of scaling? | | A3: | The commands that should be used to determine whether there is a problem are as follows: -
- show ip bgp neighbors -
- show ip bgp paths -
- show ip bgp neighbors -
- show ip bgp summary -
- show ip bgp -
- show processes cpu |
Scenario 15-2 Answers | 1: | Given the description of the company and with reference to Figure 15-12, do you agree that BGP is a requirement for this network? Give reasons for your answer. | | A1: | Because the company is small and has only a single connection per site into the Internet, it would be too complex to configure and maintain BGP when there simply are not enough resources. The bandwidth is inadequate for the task, and the administrative expertise is already overstretched. It would be far better to configure one or two static/default routes to the Internet and to redistribute these routes into the IGP running within the autonomous system. | | 2: | What alternatives are available? | | A2: | The only real alternative is the one already mentioned: to configure a default route into the Internet from every location and to redistribute this default route into the IGP for the autonomous system. | | 3: | Give the alternative configuration commands for the satellite site to connect to Internet. | | A3: | Each site would have the same configuration structure, although the details might differ : ip route 0.0.0.0 0.0.0.0 155.94.83.1 router ospf 100 network 207.111.9.0 0.0.0.255 area 0 default-information originate always The first line configures the default route. The second line turns on the OSPF process 100. The third line identifies which interfaces are participating in OSPF and what area they are in. The fourth line propagates the default route into the network, whether or not the advertising router has a path to the network. | | 4: | What commands would show that the link is up and operational? | | A4: | The commands to prove that the link is up and operational would be the show ip route command and the ping and traceroute commands. Refer to the Cisco web site or the ICND course for more details on these commands. |
Scenario 15-3 Answers | 1: | How many sessions are active? | | A1: | There are two active sessions. In reading the large amount of information on the show ip bgp neighbor command, there is a line at the beginning of each session identifying the neighboring peer. The lines in this output screen are as follows: BGP neighbor is 155.94.83.1, remote AS 2914, external link BGP neighbor is 144.39.228.49, remote AS 701, external link | | 2: | What is the state of the sessions, and what do the states mean? | | A2: | BGP state = Established, table version = 457046, up for 1w5d BGP state = Established, table version = 457055, up for 2w0d Both the peers have established sessions. This means that they have a TCP session between them. They are now in a position to exchange routing tables and to synchronize their databases. The rest of the line indicates how many times the table has been updated and how long the session has been maintained . In this example, the first peer has had a session with the local router for one week and five days, while the second peer has been up for exactly two weeks. There are three autonomous systems in this configuration. The first peer belongs to autonomous system 2914, and the second belongs to 701. Because both of these neighbors belonging to their autonomous systems have an external BGP session, there must be a third autonomous system, within which the local router resides. BGP neighbor is 155.94.83.1, remote AS 2914, external link BGP neighbor is 144.39.228.49, remote AS 701, external link |
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