![]() ![]() |
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:
In a very large internetwork, changes are inevitable.
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,
The hierarchical topology possibilities of OSPF have the following important advantages:
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.
|