Designing Implementing an OSPF Network | OSPF Network Design Solutions

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Plan Now for OSPF Summarization

The operation and benefits of route summarization have been discussed in previous chapters. At this point though, you should realize the importance of proper summarization on your network. The OSPF network in Figure 7-12 does not have summarization turned on. Notice that by not using summarization, every specific-link LSA will be propagated into the OSPF backbone and beyond, causing unnecessary network traffic and router overhead. Whenever an LSA is sent, all affected OSPF routers will have to recompute their LSA database and routes using the SPF algorithm.

Figure 7-12 No route summarization will cause network problems.

OSPF will provide some added benefits if you design the network with summarization. For example, only summary-link LSAs will propagate into the backbone (area 0). This is very important because it prevents every router from having to rerun the SPF algorithm, increases the network’s stability, and reduces unnecessary traffic. Figure 7-13 demonstrates this principle.

Figure 7-13 Proper route summarization improves OSPF network stability.

IP addresses in an OSPF network should be grouped by area, and you can expect to see areas with some or all of the following characteristics:

• Major network number(s)

• Fixed subnet mask(s)

• Random combination of networks, subnets, and host addresses

It is important that hosts, subnets, and networks be allocated in a controlled manner during the design and implementation of your OSPF network. The allocation should be in the form of contiguous blocks that are adjacent so OSPF LSAs can easily represent the address space. Figure 7-14 shows an example of this.

Figure 7-14 Configure OSPF for summarization.

TIPS:
Allocation of IP addresses should be done in powers of two so that these “blocks” can be represented by a single summary link advertisement. Through the use of the area range command you will be able to summarize large contiguous blocks of addresses. In order to minimize the number of blocks you should make them as large as possible.

Bit Splitting

Bit splitting is also a very useful technique discussed in previous chapters, and you might now want to consider using it if you have to split a large network number across more than one OSPF area. Simply put, bit splitting borrows some subnet bits for designated areas, as discussed in Chapter 5, “The Fundamentals of OSPF Routing & Design.”

To differentiate two areas, split one bit.

To differentiate 16 areas, split four bits.

Figure 7-15 demonstrates this bit splitting technique.

Figure 7-15 Bit splitting address space.

The example uses four bits for the area and uses 32-bit numbers to represent four of the 16 possible areas. The area numbers appear in dotted decimal notation and look like subnet numbers. In fact, the 32-bit area numbers correspond to the summary advertisement that represents the area.

Map OSPF Addresses for VLSM

Variable-length subnet masking (VLSM) has been discussed previously, so this section will not dwell on it too deeply. But suffice it to say that the reasons behind it are similar to bit splitting. Remember to keep small subnets in a contiguous block and increase the number of subnets for a serial meshed network. Figure 7-16 provides a good example of VLSM OSPF mappings.

Figure 7-16 VLSM OSPF mappings.

Discontiguous Subnets

Subnets become discontiguous when they are separated by one or more segments represented by a different major network number. Discontiguous subnets are supported by OSPF because subnets masks are part of the link-state database.

Consider the following example: The OSPF backbone area 0 could be a class C address, while all the other areas could consist of address ranges from a class B major network as illustrated in Figure 7-17.

Figure 7-17 OSPF network with discontiguous subnets.

TIPS:
OSPF supports discontiguous subnets regardless of whether summarization is configured within the network. Although, everything within your network will route better and have a more stable design if summarization is configured.

Naming Schema

The naming scheme used in your network should also be designed in a systematic way. By using common prefixes for names within an organization, you will make the network much easier to manage and more scalable. All of this is shown in Figure 7-11.

It is also important to carry a naming convention into your routers as well. This will assist everyone dealing with your network because the router names actually hold some meaning, instead of an abstract like an order number.

Step 4: Provision the Hardware

In Step 4, you must use vendor documentation, salesmen, and system engineers to determine the hardware necessary for your network. This is for both LAN and WAN components.

For LANs, you must select and provision router models, switch models, cabling systems, and backbone connections.

For WANs, you must select and provision router models, modems, CSUs/DSUs, and remote access servers.

Notes:
What do you know? Coming into Step 4 you have determined your network requirements, developed a physical network topology, and laid out your addressing and naming scheme for the network. In this step, you will begin selecting and provisioning the necessary network equipment to implement the design.

When selecting and provisioning routing or switching hardware, consider the following areas:

• Expected CPU usage

• Minimum RAM

• BUS budget

• Forwarding budget

• Required Interface Types and Density

Step 5: Deploy Protocol and IOS Features

In Step 5, you will need to deploy the more specific features possible by the OSPF protocol and the routers IOS. It is not necessary to have a network with every single option turned, nor is it something you are likely to see. Some of the features you should consider implementing are covered in the two sections that follow.

Notes:
What do you know? Coming into Step 5 you have determined your network requirements, developed a physical network topology, laid out your addressing and naming scheme, and begun the provisioning of the network equipment. In this step, you will begin deploying the OSPF and IOS features that you will be using within the network.

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