Configuring Route Maps for Redistribution

15 Mar

Configuring Route Maps for Redistribution
Although the filtering discussed in Chapter 17, “Implementing Redistribution and Controlling Routing Updates,” is perfectly adequate for simply denying or permitting routes from entering another routing process, route maps can do more. Their strength lies in their ability to change the route in some way. A common manipulation of the route using route maps is to change the metric. As you saw in Chapter 17, changing the metric is necessary so that the receiving routing protocol can forward the route using a metric that it understands.

The following commands are summarized here into groups: the match commands that can be configured for redistribution, and the set commands that can be applied if the route matches the criteria stated.

The match Commands for Redistribution with Route Maps The match commands used in redistribution are summarized in Table 18-5. These match commands are used to determine whether the route is to be redistributed.

Table 18-5 The match Commands Used in Redistribution

Table 18-5 The match Commands Used in Redistribution (Continued)

NOTE If a route is not matched, it is not redistributed.

The set Commands for Redistributing with Route Maps
The following set commands are used after the match criteria have been satisfied. Whereas the match parameter determines whether the route will be redistributed, the set parameter determines how the route is to be redistributed.

The set command is as follows:
Router(config-route-map)#sseett { criteria}

The set commands used in redistribution are summarized in Table 18-6.
Table 18-6 The set Commands Used in Redistribution

Once configured, the route map must be called into service. Until it is called, it has no power. The command used to recruit the services of the route map for redistribution is the redistribution command itself. Once configured, redistribution sends routes to the route map.

Router(config-router)#redistribute protocol [ process-id] [route-map map-tag]

map-tag is the name of the route map to use for redistribution. This must match a map tag specified by a route-map command.

Example 18-3 is very simple, but it clearly illustrates the functionality of the route map. Study the example in reference to Figure 18-1.

This route map examines all updates from RIP and redistributes those RIP routes with a hop count equal to 3 into OSPF. These routes will be redistributed into OSPF as external link-state advertisements (LSAs) with a metric cost of 6, a metric type of Type 1, and a tag equal to 1.

The tag is useful for tracking routes during redistribution, when the routes change from one routing domain to another, for example, from RIPv2 to OSPF. The routes are tagged at the point at which they are redistributed into another protocol. Although the routing protocols do not use the tags, they are passed between the different domains during redistribution.

Figure 18-1 Route Map to Distribute RIPv2 into OSPF

Example 18-3 Route Map to Distribute RIPv2 into OSPF

Monitoring the Configuration of Route Maps, Policy-Based Routing, and Redistribution
Most of the appropriate commands in tracking route maps are the same as those shown in Chapter 17. The commands used to test connectivity throughout the network include the following:

■ show ip protocol
■ show ip route
■ show ip route routing-protocol
■ show ip eigrp neighbors
■ show ip ospf database

In addition to these commands, trace and extended ping are also very useful. ping is particularly useful in policy-based routing where packets are routed based on packet length.

To monitor the policy-based-routing configuration, use the following EXEC commands described in Table 18-7.

Table 18-7 Commands to Monitor Policy-Based Routing

CAUTION Because the debug ip policy command generates a significant amount of output, use it only when traffic on the IP network is low so that other activity on the system is not adversely affected. This is true of all debug commands.

Foundation Summary
The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

The characteristics of route maps are summarized in the following list:

■ A route map has a list of criteria, stated with the match statement.
■ A route map can change packets or routes that are matched by using the set statement.
■ A collection of route map statements that have the same route map name are considered one route map.
■ The route map will stop as soon as a match is made, just like an access list does.
■ Within a route map, each route map statement is numbered with sequence numbers and, therefore, can be edited individually.
■ Route maps can use IP standard or extended access lists to establish policy-based routing.

— A standard IP access list can be used to specify match criteria for the source address of a packet.
— Extended access lists can be used to specify match criteria based on source and destination addresses, application, protocol type, TOS, and precedence.

■ The match route map configuration commands are used to define the conditions to be checked.
■ The set route map configuration commands are used to define the actions to be followed if there is a match.
■ A route map can contain logical ANDs as well as logical ORs.
■ The sequence number is used to specify the order in which conditions are checked. Thus, if there are two statements in a route map named BESTEST, one with sequence 5 and the other with sequence 15, sequence 5 is checked first. If there is no match for the conditions in sequence 5, then sequence 15 will be checked.

The following characterize the operation of route map statements:

■ The route map statements used for policy-based routing can be marked as permit or deny.
■ Only if the statement is marked as permit and the packet meets the match criteria will the set commands be applied.

■ The statements in a route map correspond to the lines of an access list. Specifying the match conditions in a route map is similar to specifying the source and destination addresses and masks in an access list.
■ The statements in the route map are compared to the route or packet to see if there is a match. The statements are examined in turn from the top, like in an access list. The single match statement can contain multiple conditions. At least one condition in the match statement must be true. This is a logical OR.
■ A route map statement can contain multiple match statements. All match statements in the route map statement must be considered true for the route map statement to be considered matched. This is a logical AND.

The route-map command syntax is shown here:
Router(config)#route-map map-tag [{permit | deny} sequence-number]
Table 18-8 describes the syntax options available for the route-map command.
Table 18-8 The route-map Command Options

The match commands used in policy-based routing and redistribution are summarized in Table 18-9.
Table 18-9 The match Commands Used in Policy-Based Routing

The set commands used in policy-based routing are summarized in Table 18-10.
Table 18-10 The set Commands Used in Route Maps

Figure 18-2 and Figure 18-3 summarize the logic used when route maps are applied to a router. Remember that in Figure 18-3, if there is no match or the match is a deny, the packet is not discarded but sent to the routing process to be routed by destination. If, however, the routing table has no entry for the destination, the packet will be dropped. This is not a function of route maps but rather the normal routing process.

Figure 18-2 Route Map Logic for Policy-Based Routing 1

Figure 18-3 Route Map Logic for Policy-Based Routing 2

Q&A
As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certifications,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD-ROM.

1. Explain the command match ip address {access-list-number | name} [access-list number | name].
2. Explain the command ip route-cache policy .
3. State two benefits of using policy-based routing.
4. How are matching routes modified in a route map?
5. Explain the command set ip default next-hop [ip-address...ip-address].
6. Which command displays route maps that are configured on interfaces?
7. What command is used to attach a route map to an incoming interface?
8. What is a map tag?
9. Explain briefly the difference between the match and set commands.
10. What are the criteria by which policy-based routes are determined?
11. How would you block traffic that found no match?
12. What is the purpose of the sequence number in a route map?
13. What logic is used if there are multiple match statements in the route map?
14. What parameters can an extended access list define in the route map selection process?
15. How can policy-based routing save money for the network?
16. What are some of the potential disadvantages of using policy-based routing?
17. Explain the use of the command match length min max.
18. Explain the following sample configuration.
19. Briefly explain the use of the command show ip policy .
20. Configuring route maps is complex, and it is easy to confuse the logic by which they work. State one of the things you should be aware of when configuring a route map.

Scenarios
The following scenarios and questions are designed to draw together the content of the chapter and to exercise your understanding of the concepts. There is not necessarily a right answer. The thought process and practice in manipulating the concepts are the goals of this section. The answers to the scenario questions are found at the end of this chapter.

Scenario 18-1
The hospital Duddleduddle needs to implement a routing policy using route maps. This is to ensure the optimum use of bandwidth. The X-ray department requires an enormous amount of bandwidth when transferring MRI (Magnetic Resonance Imaging) images to the centralized database.

Using Figure 18-4 and given the criteria within the questions, configure the route maps on Router A.

1. The hospital policy states that the FTP traffic from the X-ray department (201.77.11.0/24) should be forwarded to the Biggun Server at 201.77.12.79 and that it should be sent across the leased line, which is a T1 connection. What would the configuration look like?
2. The Telnet sessions and e-mail connections should be sent across the Frame Relay link. This traffic is from the same department (201.77.11.0.0/24) and is connecting to the same server. What would the configuration look like?
3. What commands would you use to verify that the policy-based routing is configured correctly and operating normally?

Figure 18-4 Route Maps Example for Scenario 18-1

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 to 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 18-1 Answers
1. The hospital policy states that the FTP traffic from the X-ray department (201.77.11.0/24) should be forwarded to the Biggun Server at 201.77.12.79 and that it should be sent across the leased line, which is a T1 connection. What would the configuration look like?

The configuration would be as follows:

2. The Telnet sessions and e-mail connections should be sent across the Frame Relay link. This traffic is from the same department (201.77.11.0.0/24) and is connecting to the same server. What would the configuration look like?

The configuration would be as follows:

Remember that if there is no match or the match is a deny, the packet is not discarded but sent to the routing process to be routed by destination. If, however, the routing table has no entry for the destination, the packet will at this point be dropped. This is not a function of route maps but rather of the normal routing process.

3. What commands would you use to verify that the policy-based routing is configured correctly and operating normally?

The commands that should be used to verify the policy-based routing are as follows:
— show ip policy
— show route-map name
— debug ip policy

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