OSPF Scalability

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
Categories of Multi-Area Components
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gives us the ability to take a large OSPF topology and break it down into multiple, more manageable
areas, as illustrated in Figure 6.1.
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, because routes can be summarized at area
boundaries, the routing tables on each router need not be as large as they would be in a singlearea
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 learning strategies for effectively troubleshooting multi-area
OSPF networks.