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Advanced OSPF Design Concepts
Life: Be glad of life because it gives you the chance to love and to work and to play and to look up at the stars.Henry Van Dyke
This chapter covers the following topics in-depth:
- OSPF redistribution
- Designing OSPF on-demand circuits
- OSPF configuration commands
- OSPF error messages
Redistribution is probably one of the most talked about and used but least understood concepts in routing. Redistribution is the process through which routing information discovered by one routing protocol is to be distributed in the update messages of another routing protocol. There are not very many networks left that do not perform some type of redistribution. A series of golden rules to follow is presented so that you may seamlessly learn the concepts behind this complicated task. This chapter will discuss several examples and techniques on how to avoid the dreaded formation of routing loops within an OSPF network. After you understand redistribution, this chapter will discuss how to perform mutual redistribution between multiple protocols.
One of the most under-funded routes, although it is just as mission critical as the networks themselves, is back up routes. Although many of todays networks have become critical to the success and operation of many corporations both large and small, these requirements often go overlooked until its too late. This chapter will discuss the golden rules for designing demand circuits and go through several design scenarios that demonstrate both good and bad designs.
The section on OSPF configuration commands was an absolute joy to include and to write. Many networking professionals are distinguished by their manuals or stacks of compact discs detailing the various router commands. In this section, the objective was to include every OSPF configuration-related command from whatever source was available and then compile them into one section, complete with examples and explanations of their uses. This section is intended to be the one stop for anyone configuring an OSPF network.
Everyone also has encountered error messages. But what do you do with them while troubleshooting an OSPF problem? Typically, they are filed away for later use or you memorize them. This section will be a boon for those who would rather memorize other things like anniversaries. During the configuration of your OSPF network, you will encounter error messages. The section, OSPF Error Messages, will help you decipher these messages and to use the necessary configuration commands to correct the problem.
Most, if not all, OSPF networks are going to have to perform redistribution at some point in their evolution. Whether you are converting a RIP network to OSPF or you are using BGP and OSPF together, the ability to accurately and effectively redistribute routes is going to be needed. This section will discuss a variety of the different issues surrounding redistribution.
Redistributing Routes into OSPF
The redistribution of routes in networking is the process of taking routes from one routing protocol and allowing a different protocol to distribute them as new routes. Figure 6-1 illustrates this concept.
Figure 6-1 Redistribution of routing information.
Redistribution consists of more than just translating routes between protocols. In addition to routes, metrics and routing updates need to be shared to ensure accurate routing.
Golden Rules of Redistributing OSPF Routes
In general, redistribution with any type of protocol can be very tricky. The following list highlights some of the more important rules to follow when redistributing routes in an OSPF network.
- The most important rule is to NEVER allow routes from one protocol to be redistributed back into the originating protocol. The results of this are discussed later in this chapter.
- On an autonomous system border router (ASBR), use the distribute-list out command to filter redistributed routes into other protocols.
- The command distribute-list in stops routes from being inserted in routing tables, but it does NOT stop link-state advertisements (LSAs) from being sent. As a result all downstream routers will learn about the networks that were supposed to be filtered by these LSAs.
- Try to avoid using filters (access lists) under the Router OSPF section of the routers configuration.
- Whenever you redistribute OSPF into other protocols, you have to respect the rules of operation for those other protocols.
Redistributing routes into OSPF from other routing protocols or from static routes will cause these routes to become OSPF external routes. To redistribute routes into OSPF, use the redistribution command in the router configuration mode. Several examples are given to further assist in demonstrating the concept of redistribution.
Example #1: Redistributing RIP into OSPF
This example illustrates a common occurrence and that redistributing RIP into OSPF as shown in Figure 6-2. The router configuration commands are also included to further illustrate this example.
Figure 6-2 Redistributing RIP into OSPF.
Router A(config)# router ospf 10 redistribute rip subnets metric-type 1 metric 12 network 220.127.116.11 router rip network 18.104.22.168 passive interface s 0 default-metric 5
All the configuration commands used will be discussed in context later in this chapter in the section titled, OSPF Configuration Commands.
Example #2: Redistributing IGRP into OSPF
This example illustrates how to redistribute IGRP into OSPF as shown in Figure 6-3. The router configuration commands are also included to further illustrate this example.
Figure 6-3 Redistributing IGRP into OSPF.
Router A(config)# router ospf 10 redistribute igrp subnets metric-type 1 metric 12 network 22.214.171.124 router igrp network 126.96.36.199 passive-interface s 0 default-metric k1 k2 k3 k4 k5
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