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 16-1 Answers

1:	Give reasons to support Humugos's desire to have multiple connections to the Internet.
A1:	Multiple connections to the Internet not only would provide redundancy, but also could be configured to load balance traffic into the Internet. If load balancing is not an option because the multiple connections are to different ISPs, traffic management could still be enforced by using each link for different purposes. Tuning the attributes and configuring prefix lists would do this very effectively.
2:	The company has been advised to redistribute static routes into the Internet BGP. It had intended to redistribute dynamic OSPF routes directly into the ISP provider. Explain why the ISP was not in favor of this configuration.
A2:	If the ISP accepted routes that had been dynamically redistributed into its autonomous system from OSPF, it could have a very unstable network. The problem is that every time there is a change anywhere that results in an update being generated by OSPF, it is redistributed into BGP, requiring BGP to process this change and generate an update. The probability is that no aggregation is configured, which leads to additional traffic and large routing tables. The last problem is that any error experienced by OSPF propagates into BGP and can cause unstable routing tables.
3:	Using Figure 16-11, issue the configuration commands that would allow Router B connecting into the Internet to select the path to network 78.9.80.0 via Router G. Use the local preference attribute to select the path .
A3:	Given the design of the network, the path to network 78.9.80.0 has a longer AS_Path through Router G. To tune the local preference to select this path means altering the selection that it would naturally have taken. The configuration commands are as follows: Router B(config)# router bgp 100 network 167.55.0.0 mask 255.255.0.0 neighbor 131.171.67.8 remote-as 100 neighbor 201.101.9.8 remote-as 250 bgp default local-preference 250 Remember that the higher the preference, the more likely the selection.

Scenario 16-2 Answers

1:	The original design required a fully meshed iBGP network. This was calculated to mean 250 connections, which was deemed unacceptable because it would be too great of a drain on resources. Route reflectors are obviously the answer. Configure Router A to run BGP and act as a route reflector to clients B and C.
A1:	router bgp 100 network 10.0.0.0 network 131.108.0.0 neighbor 10.10.10.10 remote-as 100 neighbor 10.10.10.10 route-reflector-client neighbor 10.10.8.9 remote-as 100 neighbor 10.10.8.9 remote-as 100 route-reflector-client
2:	Configure Routers B and C to run BGP as clients to Router A.
A2:	Router B: router bgp 100 network 10.0.0.0 neighbor 10.10.10.9 route-reflector-client Router C: router bgp 100 network 10.0.0.0 neighbor 10.10.8.10 remote-as 100
3:	Having configured the cluster, are any other tasks necessary?
A3:	Given that the route reflector is now forwarding the routes between B and C, the link between these routers is no longer necessary, and the BGP link between them should be broken. This simply requires the removal of the neighbor statements that create the link on both Routers B and C.
4:	The company has decided in its early stages to require organizations connecting into it to use default routes. How would these routes be disseminated without the organization's autonomous system, and how would an interior router running only an IGP determine which path out of the routing domain to take if it had more than one connection?
A4:	The routers in the client organization do not need to run BGP. They simply need to configure a default route and propagate this into the routing domain, in accordance with the interior routing protocol that is being run. If the autonomous system were multihomed into the Internet, there would be more than one default route propagated throughout the system. Any router within the autonomous system would determine the best path to the outside world by comparing the routing protocol metrics between the default routes. Thus, RIP would select the lowest hop count, EIGRP the lowest combination of bandwidth and delay, and OSPF the lowest cost.
5:	Given that the use of a default route by the client organization gives it the least configuration power to manage and manipulate its traffic flow, how would the network administrator justify this simple approach?
A5:	The default route, although giving the least control over the connection to the Internet, is very robust in that it has no working parts to fail. Therefore, it requires very little CPU or memory. The lack of redistribution eliminates the possibility of routing loops , and the lack of a routing protocol running over the physical link to the Internet frees up bandwidth for data.

Scenario 16-3 Answers

1:	Using Example 16-10, identify how many times the route 44.0.0.0/8 has been sent in outgoing updates from the router.
A1:	The prefix list tryout has 28 hits logged for the network 44.0.0.0/8. This means that 28 updates have been sent with the network 44.0.0.0 from the router to its neighbors.
2:	Which path will be chosen in Example 16-11 to get to 33.48.124.0, and why?
A2:	The path using 144.39.228.49 as the next hop will be used. The local preference is set to 200, because the local preference prefers a higher value.
3:	What is the most likely reason for the source of a route to be flagged as incomplete?
A3:	The route was probably redistributed into BGP, and it therefore cannot identify as much information as if it were received as a routing update with attributes attached.
4:	To send packets to network 7.2.0.0/16, the router will direct traffic to a next-hop router. The data frame at Layer 2 will be addressed to this next hop, which will route it on to the next router in the journey to its destination. What is the Layer 3 address of the next logical hop, and why was it selected?
A4:	The next logical hop for the route 7.2.0.0/16 is 144.39.228.49. This address was selected because it is the next hop in the best path to the destination. BGP determined the best path based on AS_Path. The alternate route has to journey through two autonomous systems to find the destination network, so this path has a more direct route. Because neither the weight attribute nor the local preference attribute has been tuned , the AS_Path is the determining attribute. This information is not shown in the output screen.