Your ability to scale an OSPF internetwork depends on your overall network structure and IP addressing scheme. Adopting a hierarchical network design with a structured address assignment (that is, using summarization whenever possible) is the most important factor in determining the overall scalability of your OSPF network. Network scalability is affected by both operational and technical considerations.
Operationally, OSPF networks should be designed so that areas do not need to be split to accommodate predicted and unpredicted growth. Networks will most likely not shrink in size, so plan accordingly. Specifically, reserve IP address space to permit the addition of new routers and areas.
Scalability should always be taken into consideration when designing your network. All routers keep a copy of the area's link-state database (LSDB). If a router is in more than one area, such as an ABR, the router has one LSDB for each area. As your network grows, its size eventually reaches a point where the database becomes too large, resulting in routing inefficiencies because the router does not have the resources to handle normal routing activities.
The larger the OSPF area, the more LSAs are flooded throughout the network whenever there is a topology change. This can result in network congestion. It is impossible to predict whether a congestion problem will result in normal LSA flooding. The factors involved are too extensive to accurately predict. In modern networks, congestion-causing LSAs are rare. However, too many LSAs causing slow SPF calculations is quite common-especially LSAs from external routes being flooded across the entire AS.
190 Chapter 4: Design Fundamentals
The capability of your OSPF network to scale properly is determined by a multitude of factors, including the following:
• OSPF functional requirements (that is, what a router needs to do)
• Router memory requirements
• CPU requirements
• Available bandwidth
• Security requirements
NOTE In many cases, personnel who work directly with networks are not always in complete control of some of the factors discussed in this section. Of course, bigger routers are better; unfortunately, management does not always allow you to purchase the biggest routers. In the end, a compromise is usually required. Be sure to document the limitations and potential growth issues with the compromise solution that you implement.
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