Chapter 5 Interconnecting OSPF Areas THE CCNP ROUTING EXAM TOPICS COVERED IN THIS CHAPTER ARE AS FOLLOWS OSPF scalability considerations Definitions of multi area components e g classifications of routers Link State Advertisements and areas Step by step guide to multi area OSPF configuration Guidelines for establishing stub totally stubby and not sostubby areas Virtual link configuration Strategies for monitoring and troubleshooting multi area OSPF networks Copyright 2001 SYBEX Inc Alameda CA www sybex com I n this chapter we will illustrate the scalability constraints of an OSPF network with a single area The concept of multi area OSPF will be introduced as a solution to these scalability limitations This chapter will also identify the various categories of routers used in multi area configurations These router categories include a backbone router internal router area border router ABR and autonomous system boundary router ASBR We ll explore how these routers can use summarization and default routes to reduce the amount of route information that is injected into an area thus reducing a router s memory and processor overhead The functions of different OSPF Link State Advertisements LSAs are very important to understand for the Routing exam and we will detail the types of LSAs used by OSPF We will see how these LSAs can be minimized through the effective implementation of specific OSPF area types Specifically we will examine stub areas totally stubby areas and not sostubby areas and show how these areas can be used to minimize the number of LSAs advertised into an area We ll also provide a set of design guidelines and configuration examples as well as the syntax required to configure route summarization at both area border routers and autonomous system boundary routers You ll learn that all areas need to have a link to Area 0 If an area is not attached to Area 0 virtual links can be used to span transit areas in OSPF networks where all areas are not physically adjacent to the backbone area We then will conclude with a collection of debug and show commands that can be used to effectively monitor and troubleshoot a multi area OSPF implementation Copyright 2001 SYBEX Inc Alameda CA www sybex com OSPF Scalability 167 OSPF Scalability In the previous chapter we examined the configuration of OSPF networks that contained a single area We saw that OSPF had significant advantages over distance vector protocols such as RIP due to OSPF s ability to represent an entire network within its link state database thus vastly reducing the time required for convergence However let s consider what the router does in order to give us such great performance Each router recalculates its database every time there is a topology change requiring CPU overhead Each router has to hold the entire link state database which represents the topology of the entire network requiring memory overhead Furthermore each router contains a complete copy of the routing table requiring more memory overhead Keep in mind that the number of entries in the routing table may be significantly greater than the number of networks in the routing table because we may have multiple routes to multiple networks With these OSPF behavioral characteristics in mind it becomes obvious that in very large networks single area OSPF has some serious scalability considerations Fortunately OSPF gives us the ability to take a large OSPF topology and break it down into multiple more manageable areas as illustrated in Figure 5 1 FIGURE 5 1 OSPF areas Multi area OSPF network Single area OSPF network Area 0 Area 0 Copyright 2001 SYBEX Inc Alameda CA Area 10 www sybex com Area 20 168 Chapter 5 Interconnecting OSPF Areas Consider the advantages of this hierarchical approach First of all routers that are internal to a defined area need not worry about having a link state database for the entire network only their own areas thus reducing memory overhead Second routers that are internal to a defined area now only have to recalculate their link state database when there is a topology change within their particular area Topology changes in one area will not cause global OSPF recalculations thus reducing processor overhead Finally since routes can be summarized at area boundaries the routing tables on each router need not be as large as they would be in a single area environment Of course as we start subdividing our OSPF topology into multiple areas we introduce some complexity into our configuration Therefore in this chapter we will examine these various configuration subtleties in addition to strategies for effectively troubleshooting multi area OSPF networks Categories of Multi area Components T his section covers the various roles that routers play in an OSPF large network These include backbone routers internal routers area border routers and autonomous system boundary routers We ll also discuss the different types of advertisements that are used in an OSPF network and the different types of areas that can be configured OSPF Router Roles As we alluded to earlier routers within a multi area OSPF network fall into different categories To gain an understanding of the various roles that our routers can play let s consider Figure 5 2 Copyright 2001 SYBEX Inc Alameda CA www sybex com Categories of Multi area Components FIGURE 5 2 169 Router roles EIGRP Autonomous System Internal Router Backbone Router Internal Router Area 10 RouterC Area 0 RouterB RouterA Area Border Router Autonomous System Boundary Router Backbone Router Starting at the core of the given network and working our way outward consider RouterA Notice that RouterA is part of Area 0 As we learned in the previous chapter Area 0 is referred to as the backbone area Therefore we can make the following definition Backbone router A backbone router is any router that exists wholly or in part in OSPF Area 0 Another distinction that we can make about RouterA is that it is contained completely within a single area in this case Area 0 Since all of RouterA s interfaces are internal to a single area we can make the following definition Internal router An internal router is any router that has all of its interfaces as members of the same area Remember that a router can play more than one role In our example RouterA is both a backbone router and an internal router Now consider RouterB Notice that RouterB meets the requirement to be classified as a backbone router i e RouterB has one or more interfaces that are