OSPF Network Scalability

Last Updated on Mon, 16 Jan 2023 | Ospf Network

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.

Continue reading here: Selecting the Designated Router

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Readers' Questions

  • carlos krug
    How to scale ospf network?
    1 year ago
    1. Utilize multiple areas: Breaking the network into multiple OSPF areas can reduce the size of the area summary LSAs, thereby decreasing the size of the database and the amount of flooding across the network.
    2. Adjust OSPF Hello Timers: By increasing the OSPF Hello and Deadline timer settings, the amount of LSAs being sent on the network can be reduced and the amount of flooding can be decreased.
    3. Tune OSPF Link Cost Settings: If a network designer modifies the cost of a link (in terms of the bandwidth of a link or any other measure) the traffic across the network can be efficiently routed.
    4. Use OSPF Stub and Totally Stubby Areas: The utilization of OSPF stub and totally stubby areas will reduce the amount of LSAs sent across the network and the associated flooding.
    5. Utilize External Routes: External routes can be used to reduce routes being sent out in the database. Utilizing external routes can also eliminate the need for summarization.
    6. Utilize NSSAs (Not-So-Stubby Areas): Non-stub areas allow external routes to be advertised while not requiring the use of summarization. This can help reduce the amount of database traffic and flooding.