The Fundamentals of OSPF Routing Design

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Fully Meshed Versus Partially Meshed Network Topology

Nonbroadcast multiaccess (NBMA) clouds, such as Frame Relay or X.25, are always a challenge. The combination of low bandwidth and too many LSAs is a recipe for problems—even for OSPF. A partially-meshed topology has been proven to behave much better than a fully-meshed network topology. Figure 5-4 shows the benefits and differences between the two topologies.

Figure 5-4  Examples of fully- and partially- meshed networks.

A carefully laid out point-to-point or point-to-multipoint network can in some cases work much better than multipoint networks that have to deal with DR issues.

The Link-State Database

Although covered in previous chapters, these issues as related to the LSDB are very important and deal directly with its operation in relation to the topology of the network.

  A router has a separate LSDB for each area to which it belongs.
  All routers belonging to the same area have the identical LSDB.
  SPF calculation is performed separately for each area and its associated LSDB.
  LSA flooding occurs only within the area sending the advertisement.

OSPF Network Scalability

Your ability to scale an OSPF internetwork depends on your overall network structure and IP addressing scheme. As outlined in the discussions concerning network topology and route summarization, adopting a hierarchical addressing environment and a structured address assignment will be the most important factors in determining the scalability of your internetwork. 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 growth. Address space should be reserved to permit the addition of new areas. Scalability should always be taken into consideration when designing your network. All routers keep a copy of the LSDB. As the network grows, they will eventually reach a point where the database becomes too large, resulting in inefficiency in your routing. Additionally, the LSAs will be flooded throughout the network, resulting in a congestion problem. The capability of your OSPF network to scale properly is determined by a multitude of factors, including the following:

  Router memory requirements
  CPU requirements
  Available bandwith
  OSPF security

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 is better; unfortunately, it is also more expensive.

Determining Router Memory Requirements

An OSPF router stores all of the link states for all of the areas that it is in. In addition, it can store summary and external routes. Careful use of route summarization techniques and the creation of stub areas can reduce memory use substantially.

It is not easy to determine the exact amount of memory needed for a particular OSPF configuration. Memory issues usually come up when too many external routes are injected in the OSPF domain. A backbone area with 40 routers and a default route to the outside world would have less memory issues compared with a backbone area with 4 routers and 33,000 external routes being injected into OSPF. Router memory could also be conserved by using a good OSPF design. Summarization at the area border routers and use of stub areas could further minimize the number of routes exchanged.

The total memory used by OSPF is the sum of the memory used in the routing table (show ip route summary) and the memory used in the LSDB. The following numbers are a “rule of thumb” estimate. Each entry in the routing table will consume between approximately 200 and 280 bytes plus 44 bytes per extra path. Each LSA will consume a 100 byte overhead plus the size of the actual LSA, possibly another 60 to 100 bytes (For router links, this depends on the number of interfaces on the router). These amounts should be added to memory already used by other processes and by the IOS itself.

If you really want to know the exact number, you can do a show memory with and without OSPF being turned on. The difference in the processor memory used would be the answer.

Consider getting and keeping available a backup copy of the router configuration beforehand, of course.

Normally, a routing table using less than 500K bytes could be accommodated with 2 to 4MB of RAM; large networks that have routing tables greater than 500K might need 8 to 16MB. They might even need 32 to 64MB if full routes are injected from the Internet.

CPU Requirements

An OSPF router uses CPU cycles whenever a link-state change occurs. Thus, keeping the OSPF areas small and using route summarization dramatically reduces usage of the router’s CPU and creates a more stable environment within which OSPF can operate.

Available Bandwidth

OSPF sends partial LSA updates when a link-state change occurs. The updates are flooded to all routers in the area. In a quiet network, OSPF is a quiet protocol, go figure; aren’t all protocols that way? Sorry, it had to be said. In a network with substantial topology changes, OSPF minimizes the amount of bandwidth used for customer traffic.

OSPF Security

The two kinds of security applicable to routing protocols are as follows:

  The routers that participate in an OSPF network are controlled.
  OSPF contains an optional authentication field.

All routers within an area must agree on the value of the authentication field. Because OSPF is a standard protocol available on many platforms, including some hosts, using the authentication field prevents the inadvertent startup of OSPF in an uncontrolled platform on your network and reduces the potential for instability.

You might think it is possible to control the routing information within an OSPF area. Remember though, that for OSPF to operate properly, all routers within an area must have the same data. As a result, it is not possible to use route filters in an OSPF network to provide security because OSPF exchanges route information through the use of LSAs, not routes. OSPF then calculates the route to a destination based upon the LSA.

Area Design Considerations

When creating large-scale OSPF internetworks, the definition of areas and assignment of resources within areas must be done with a pragmatic view of your OSPF internetwork. This assignment of resources includes both physical and logical networking components so that optimal performance will result.

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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