Redistribution Between IGP and BGP

14 Mar

Redistribution Between IGP and BGP
Up until now, the discussion has been about BGP, its operation, and the configuration options available. However, for most networks, BGP is the means by which information about the Internet is brought into the internal organizational routing domain. Disseminating this information throughout the autonomous system is the subject of the next section.

If the organization is not an ISP, there is a fair chance that the network is running an IGP within the autonomous system. The IP routing table generated by this protocol or protocols is distinct from the BGP routing table, although as you have seen, they communicate freely. The use of synchronization is a case in point. However, populating one routing table with routes from the other routing table needs to be manually configured.

Routes can be injected from the IGP into BGP, or from the BGP into the IGP, as discussed in the next sections.

Advertising Routes from IGP into BGP
First, consider the advertising routes into BGP. There are three ways of populating the BGP table with IGP routes:

■ Using the network command —This is used to advertise routes that are in the IP routing table.
■ Redistributing static routes —Although any static route can be redistributed into BGP, static routes are often used to create a supernet. The static route is a summary of classful addresses, such as Class C addresses with a prefix mask of 16 bits instead of 24 bits. This requires statically routing to null 0. This fools the system by creating a route that has no exit point from the router because the route does not exist but is redistributed into BGP. The command places the route into the routing table without fear of it being used and creating a black hole.

The problem is that if the route in the IGP routing table disappears, BGP still advertises the route, causing traffic to journey into the autonomous system, only to die. Therefore, Cisco suggests that you use the aggregate-address command for BGP instead.

■ Redistributing dynamically learned routes from the IGP —This configuration is not advised because there is a great reliance on the IGP table. It is imperative that external routes carried in iBGP are filtered out; otherwise, routing loops are generated when BGP routes are fed into IGP, only to be advertised back into BGP further down the network.

Advertising Routes from BGP into an IGP
Redistributing the routes from the Internet into a small network is unwise. The proposition is alarming because of the vastness of the Internet and the enormity of the routing tables. Even with a large amount of aggregation and filtering, there is still a lot of information to carry over.

Because ISPs tend to run eBGP and iBGP extensively, ISPs often run them exclusively for exterior routes, using an IGP only for internal routes. This requires no redistribution, which is easier for the routers and means the following:

■ The resources are available for other processes.
■ The IGP routing table is spared.

The synchronization function is not necessary in this type of network because iBGP is running in a fully meshed environment. With either a fully meshed network or carefully designed route reflectors, the synchronization rule can be turned off.

If iBGP is multihomed, redistribution from BGP into the IGP is needed. The IGP needs to carry the external routes across the autonomous system to the other BGP router. Also, any device wanting to connect to the Internet needs to have either a default route or specific routes to direct traffic forward. Filtering must be configured; otherwise, the internal routing tables will become overwhelmed. This is illustrated in Figure 16-10.

Figure 16-10 Redistributing BGP Routes into a Non-ISP Organization

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.

Table 16-10 summarizes the commands you have seen throughout this chapter.
Table 16-10 Summary of Commands Used in This Chapter

The benefits of route reflectors include the following:
■ The capability to scale the network, given the other characteristics
■ A strong hierarchical design
■ A reduction of traffic on the network
■ A reduction in the memory and CPU needed to maintain TCP sessions
■ Faster convergence and a simpler network because two routing protocols are implemented:
— iBGP for external routing information traversing the autonomous system
— IGP for routes internal to the autonomous system
Characteristics of route reflectors are as follows:

■ A route reflector is a router that forwards updates to its clients. When a client sends an update to the route reflector, it is forwarded or reflected to the other clients and nonclients.
■ The route reflector is the only router that is configured or that has the remotest idea that it is anything other than a peer.
■ A client is a router that receives updates from a route reflector that a route reflector has forwarded from another client or nonclient.
■ Both a route reflector and its clients, therefore, form a unit that shares information. This unit is called a cluster.
■ The autonomous system can be divided into clusters and be configured. There must be at least one route reflector per cluster; otherwise, the clients will not get the updates reflected to them.
■ The route reflector and the client no longer require a full mesh of peering relationships because the route reflector forwards updates from other clients.
■ In all probability, a route reflector is connected to peers for whom it is not forwarding routes. These are regular neighbors or peers, but from the route reflector’s view, they are nonclients.
■ Nonclients must be fully meshed with the route reflector and each other.
■ The route reflector connects to other route reflectors. These route reflectors need to be fully meshed because the old rule of not propagating routes that are not defined in the network command is now operational. This is to ensure that the iBGP routing tables are complete.
■ When the route reflector forwards an update, the Originator-ID attribute is set. This is the BGP router ID of the router that originated the path. The purpose of this attribute is not to award honors to the originating router, but so that if this router receives the update, it will see its own ID and will ignore the packet. This prevents the possibility of routing loops.
■ If there are multiple route reflectors in the cluster to provide redundancy, then the originating router is identified by the Cluster-ID attribute. This serves the same purpose as the Originator- ID in preventing routing loops.

The rules by which route reflectors propagate updates are as follows:

■ If a route reflector receives multiple paths to the same destination, it chooses the best path.
■ If the route is received from a client, the route reflector reflects or forwards the update to clients and nonclients, except for the originator of the route.
■ If the route is received from a nonclient, the route reflector reflects the update only to clients.
■ If the route is received from eBGP, the route reflector or client reflects it to all nonclients, as well as clients.

Whether a prefix is permitted or denied is based upon the following rules:

■ If a route is permitted, the route is used.
■ If a route is denied, the route is not used.
■ At the bottom of every prefix list is an implicit deny any . Thus, if the given prefix does not match any entries of a prefix list, it is denied.
■ When multiple entries of a prefix list match a given prefix, the entry with the smallest sequence number (the first match in the list) is used.
■ The router begins the search at the top of the prefix list, with the sequence number 1. When a match is made, the search stops. Processing time will be reduced if the most common matches or denies are placed near the top of the list. This will prevent having to process criteria that are seldom met every time a route is examined.
■ Sequence numbers are generated automatically by default. To configure the sequence numbers manually, use the seq seq-value argument of the ip prefix-list command.
■ A sequence number does not need to be specified when removing a configuration entry.

Table 16-11 lists the various command options for prefix lists.
Table 16-11 Displaying Prefix List Command Options

Table 16-11 Displaying Prefix List Command Options (Continued)

The show commands always include the sequence numbers in their output.
Table 16-12 summarizes the different approaches to obtaining routing information from the Internet.
Table 16-12 Receiving Routing Updates from Multiple ISPs

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. If a route reflector hears an update from a nonclient, what action will be taken?
2. In version 11.0 of the Cisco IOS software, what method would be used to restrict routing information from being received or propagated?
3. Explain the purpose and use of the command show ip prefix-list name [seq seq-number].
4. Why would you redistribute static routes into BGP?
5. Why is it advisable to have the route reflectors fully meshed?
6. Why is filtering often required when redistributing BGP into an IGP?
7. What are the advantages of multihoming?
8. Why do iBGP peers need to be fully meshed?
9. How is a fully meshed network avoided in iBGP?
10. What is the equation to determine the number of sessions needed in a fully meshed BGP network?
11. Why does a fully meshed network in iBGP cause problems?
12. State two benefits to using route reflectors.
13. If a route reflector sees multiple paths to a destination, what action is taken?
14. Explain the difference between a cluster-ID and an originator-ID.
15. State two advantages in using prefix lists over access lists.
16. If the ISP has provided a default route, how will the router within the autonomous system select the exit path in a multihomed environment?
17. What is a disadvantage of an autonomous system receiving full routing updates from all ISPs?
18. What is the danger of redistributing BGP into the IGP?
19. What are the advantages of a fully meshed iBGP network?
20. In configuring a route reflector, how is the client configured?
21. What commands are used to display the BGP router ID that identifies the router that is sending the updates and peering with its neighbor?

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 16-1
The company Humugos has successfully implemented iBGP in each country that it operates in, with eBGP connecting the autonomous systems. The company now wants to change the way it connects to the Internet. Currently, it has one connection into the Internet per autonomous system. Figure 16-11 provides the diagram for the network in this scenario.

Figure 16-11 Diagram for Scenario 16-1

1. Give reasons to support Humugos’s desire to have multiple connections to the Internet.
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.
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.

NOTE This network scenario is oversimplified for learning purposes. Normally, it would be very difficult to obtain multiple autonomous system numbers from the Internet. Private autonomous system numbers would have to be used, which would make connections into the Internet complex.

Scenario 16-2
The ISP Interconnect Corp. is a startup company that is configuring its network. The company has a well-resourced network and is in the process of configuring the iBGP within the autonomous system. Figure 16-12 provides the diagram for the network in this scenario.

Figure 16-12 Diagram for Scenario 16-2

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.
2. Configure Routers B and C to run BGP as clients to Router A.
3. Having configured the cluster, are any other tasks necessary?
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?
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?

Scenario 16-3
Review output screens in Examples 16-10 and 16-11, and answer the questions that follow.
Example 16-10 Scenario 16-3 Output Screen 1

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.
2. Which path will be chosen in Example 16-11 to get to 33.48.124.0, and why?
3. What is the most likely reason for the source of a route to be flagged as incomplete?
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?

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.

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.
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.

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:

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 as a route reflector to clients B and C.

3. Having configured the cluster, are any other tasks necessary?

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 them 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?

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 its the least configuration power to manage and manipulate its traffic flow, how would the network administrator justify this simple approach?

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?

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?

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?

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?

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. Thi  information is not shown in the output screen.

No comments yet

Leave a Reply

You must be logged in to post a comment.