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| 6.5 |  | Multiarea OSPF Operation |
| 6.5.1 |  | Creating multiple OSPF areas |

Three issues can overwhelm an OSPF router in a heavily populated OSPF network:

- high demand for router processing and memory resources
- large routing tables
- large topology tables

In a very large internetwork, changes are inevitable. OSPF routers are likely to run SPF calculations frequently, which deprive the router of precious CPU cycles and memory resources.

Not only is the routing table frequently recalculated in a large OSPF network, but it also risks being overstuffed with multiple paths and hundreds of routes. Full routing tables make routers less efficient. Finally, the link-state database, which must contain a complete topology of the network, will also threaten to consume resources and slow down the router.

Fortunately, OSPF allows large areas to be separated into smaller, more manageable areas. These smaller areas can exchange summaries of routing information rather than exchange every detail. By splitting the network into manageable pieces, OSPF routers can scale gracefully.

Just how many routers can an OSPF area support? Field studies have shown that a single OSPF area should not stretch beyond 50 routers, although there is no set limit. Some areas may do fine with more than 50 routers. Other areas, particularly those with unstable links, may need to operate with fewer than 50 routers. Ultimately, it must be determined just how many routers a particular OSPF area can handle. Knowing the network, by tracking performance and monitoring usage, is the only way to accurately gauge whether an OSPF area can support 20, 30, or 60 routers.

The capability of OSPF to separate a large internetwork into multiple areas is referred to as hierarchical routing. Hierarchical routing enables the separation of large internetworks into smaller internetworks that are called interareas. With this technique, interarea routing still occurs. Interarea routing is the process of exchanging routing information between OSPF areas. However, interarea routing allows OSPF to summarize and contain area specific information so that many of the smaller internal routing operations, such as recalculating the database, are restricted within an area.

For example, if Area 1 is having problems with a link going up and down or flapping, routers in other areas do not need to run their shortest path first (SPF) calculation because they are isolated from the problems in Area 1.

The hierarchical topology possibilities of OSPF have the following important advantages:

- Reduced frequency of SPF calculations – Because detailed route information is kept within each area, it is not necessary to flood all link-state changes to all other areas. Therefore, only those routers affected by a change need to run the SPF calculation.
- Smaller routing tables – When using multiple areas, detailed route entries for specific networks within an area are kept inside the area. Rather than advertise these explicit routes outside the area, the routes can be summarized into one or more summary routes. Advertising these summaries reduces the amount of LSAs propagated between areas but allows all networks to remain reachable.
- Reduced link-state update (LSU) overhead – LSUs can contain a variety of LSA types, including link-state information and summary information. Rather than send an LSU about each network to every area, advertise a single route or a few summarized routes between areas to reduce the overhead associated with LSUs that cross multiple areas.

Hierarchical routing increases routing efficiency because it allows the ability to control the type of routing information that flows into and out of an area. OSPF provides for different types of routing updates, depending on the type of area and the number of areas that a router connects to. The following sections describe the different roles that an OSPF router can play, the types of LSAs that it can use, and the types of areas that it can connect to.

| Interactive Media Activity Drag and Drop: Comparison of OSPF and EIGRP Features Upon completion of this activity, the student will be able to identify and compare the different features between OSPF and EIGRP.

| Web Links OSPF Design Guide http://www.cisco.com/warp/public/104/1.html