Scenario Answers

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 15-3 as a reference, issue the commands that need to be configured at each country or autonomous system.
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 15-7 shows the configuration of the autonomous system 64522, which is the San Francisco router. It has been assigned network

Example 15-7. Scenario 15-1 Configuration

router bgp 64522
no auto-summary
neighbor remote-as 64521
neighbor remote-as 64523
neighbor remote-as 64524
neighbor remote-as 64525
neighbor remote-as 64526
neighbor remote-as 64527
neighbor remote-as 64528
neighbor remote-as 64529
neighbor remote-as 64530
!neighbor commands omitted for brevity
neighbor remote-as 64750

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 are advertised; otherwise, each subnet would need to be defined as a network command.

2.What commands would indicate that there was a problem of scaling?
The show ip bgp, show ip bgp summary, and show ip route commands would indicate whether there is a scaling issue.

Scenario 15-2 Answers

1.Given the description of the company and with reference to Figure 15-4, do you agree that BGP is a requirement for this network? Give reasons for your answer.
Because the company is small and has only a single connection per site to the Internet, it would be too complex to configure and maintain BGP when there are simply 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?
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.
Each site would have the same configuration structure, although the details might differ:
ip route
router ospf 100
network 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?
The commands to prove that the link is up and operational are 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?
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, remote AS 2914, external link
BGP neighbor is, remote AS 701, external link

2.What is the state of the sessions, and what do the states indicate?
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 that belong 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, remote AS 2914, external link
BGP neighbor is, remote AS 701, external link