This chapter covers the following subjects:
- Route Redistribution Basics: This section discusses the reasons why designers might choose to use route redistribution, and how routing protocols redistribute routes from the IP routing table.
- Redistribution in EIGRP: This section discusses the mechanics of how IOS redistributes routes from other sources into EIGRP.
- Redistribution in OSPF: This section discusses the mechanics of how IOS redistributes routes from other sources into OSPF.
This chapter begins Part 4 of this book: "Path Control." Path control refers to a general class of tools and protocols that Layer 3 devices use to learn, manipulate, and use IP routes. EIGRP and OSPF certainly fit into that category.
The three chapters in Part 4 examine path control features that go beyond the base function of learning IP routes. In particular, Chapter 9, "Basic IGP Redistribution," and Chapter 10, "Advanced IGP Redistribution," examine how routers can exchange routes between routing protocols through route redistribution. Chapter 9 begins by discussing the mechanics of what happens when the routes are redistributed. Chapter 10 then goes further to discuss how to also filter and summarize routes when redistributing–yet another path control function–and discusses some issues and solutions when multiple routers redistribute the same routes. Finally, Chapter 11, "Policy-Based Routing and IP Service Level Agreement," discusses a few smaller path control topics, including IP Service-Level Agreement (SLA) and Policy-Based Routing (PBR).
"Do I Know This Already?" Quiz
The "Do I Know This Already?" quiz allows you to assess if you should read the entire chapter. If you miss no more than one of these eight self-assessment questions, you might want to move ahead to the "Exam Preparation Tasks." Table 9-1 lists the major headings in this chapter and the "Do I Know This Already?" quiz questions covering the material in those headings so that you can assess your knowledge of these specific areas. The answers to the "Do I Know This Already?" quiz appear in Appendix A.
Table 9-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundations Topics Section
Route Redistribution Basics
Redistribution into EIGRP
Redistribution into OSPF
Which of the following answers is the least likely reason for an engineer to choose to use route redistribution?
- To exchange routes between merged companies
- To give separate control over routing to different parts of one company
- To support multiple router vendors
- To knit together an OSPF area if the area becomes discontiguous
For a router to successfully redistribute routes between OSPF and EIGRP, which of the following are true? (Choose two.)
- The router must have one routing protocol configured, but configuration for both routing protocols is not necessary.
- The router must have at least one working link connected to each routing domain.
- The redistribute command must be configured under EIGRP to send the routes to OSPF.
- The redistribute command should be configured under OSPF to take routes from EIGRP into OSPF.
Process EIGRP 1 is redistributing routes from process OSPF 2. Which two of the following methods may be used to set the metrics of the redistributed routes? (Choose 2)
- Let the metrics default.
- Set the metric components using the redistribute command's metric keyword.
- Set the metric components using the default-metric router subcommand.
- Set the integer (composite) metric using the redistribute command's metric keyword.
Examine the following excerpt from the show ip eigrp topology 10.2.2.0/24 command on router R1. Which answer can be verified as definitely true based on this output?
External data: Originating router is 10.1.1.1 AS number of route is 1 External protocol is OSPF, external metric is 64 Administrator tag is 0 (0x00000000)
- R1 is the router that redistributed the route.
- R1's metric to reach subnet 10.2.2.0/24 is 64.
- The route was redistributed on a router that has a router ospf 1 command configured.
- R1 is redistributing a route to prefix 10.2.2.0/24 into OSPF.
Router R1 has a connected route for 10.1.1.0/24 off interface Fa0/0. Interface Fa0/0 has been enabled for OSPF due to a router ospf 1 and network 10.0.0.0 0.0.0.255 area 0 command. R1 also has EIGRP configured, with the redistribute ospf 1 metric 1000 100 10 1 1500 command configured under EIGRP. Which one of the following is true?
- R1 will not redistribute 10.1.1.0/24 into EIGRP, because R1 knows it as a connected route and not as an OSPF route.
- For any OSPF routes redistributed into EIGRP, the metric components include a value equivalent to 1 Mbps of bandwidth.
- For any OSPF routes redistributed into EIGRP, the metric components include a value equivalent to 100 microseconds of delay.
- No subnets of network 10.0.0.0 will be redistributed due to the omission of the subnets parameter.
Process OSPF 1 is redistributing routes from process OSPF 2. Which of the following methods may be used to set the metrics of the redistributed routes? (Choose two.)
- Let the metrics default.
- Use each redistributed route's OSPF metric using the redistribute command's metric transparent keywords.
- Set the metric using the default-metric router subcommand.
- Redistribution is not allowed between two OSPF processes.
Examine the following excerpt from the show ip ospf database asbr-summary command on router R1 (RID 18.104.22.168). Which answer can be verified as definitely true based on this output?
LS Type: Summary Links (AS Boundary Router) Link State ID: 22.214.171.124 (AS Boundary Router address) Advertising Router: 126.96.36.199 LS Seq Number: 8000000D Checksum: 0xE43A Length: 28 Network Mask: /0 TOS: 0 Metric: 100
- The output describes the contents of a Type 5 LSA.
- 188.8.131.52 identifies a router as being the router performing redistribution.
- R1's metric for its best route to reach the router with RID 184.108.40.206 is 100.
- The router with RID 220.127.116.11's metric for its best route to reach the router with RID 18.104.22.168 is 100.
Router R1 sits inside OSPF area 1. Router R2 redistributes an E1 route into OSPF for prefix 22.214.171.124/24, with external metric 20. Router R22 redistributes an E2 route for the same prefix/length, external metric 10. Under what conditions will R1 choose as its best route the route through R22?
- R1 will always choose the route through R22.
- As long as R1's best internal OSPF cost to reach R22 is less than 10.
- As long as R1's best internal OSPF cost to reach R22 is less than 20.
- R1 will never choose the route through R22 if the E1 route through R2 is available.