Step 2 Develop the Network Topology

When designing an OSPF network, this step covers the process of determining the network's physical layout. Two common design topologies exist: meshed and hierarchical. The following sections discuss each topology and help you determine which is the most efficient design for today's networks.

What Do You Know?

Going into Step 2, you have developed a list of the requirements associated with the OSPF network. You have also begun to determine the financial costs associated with the network. These costs might include equipment, memory, and associated media.

Fully Meshed Topology

In a fully meshed structure, the topology is flat and all routers perform essentially the same function, so there is no clear definition of where specific functions are performed. Network expansion tends to proceed in a haphazard, arbitrary manner. This type of topology is not acceptable to the operation of OSPF. It does not correctly support the use of areas.

Hierarchical Topology

In a hierarchical topology, the network is organized in layers that have clearly defined functions. This type of network includes the following layers:

• Core layer—This is an excellent place for OSPF backbone routers that connect through area 0. All these routers would interconnect and without host connections. The primary purpose of the core layer is to provide connectivity between other areas.

In modern network design, the core layer can also be Gigabit Ethernet switches rather than routers.

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• Distribution layer—In this layer, you can locate other OSPF areas connected through Area Border Routers (ABRs) back to the core layer (area 0). This is also a good location to begin implementing various network policies, such as security, Domain Name System (DNS), and so on.

• Access layer—This layer includes the routers that provide connections to the users' LANs. This layer is where the majority of the hosts and servers need to connect to the network.

By using this type of logical layered design, you gain benefits that help you design the network, as shown in Figure 4-4.

Figure 4-4 OSPF Hierarchical Topology

Backbone Routers in Core Layer (Area 0)

Figure 4-4 OSPF Hierarchical Topology

Backbone Routers in Core Layer (Area 0)

The benefits of the OSPF hierarchical topology as implemented in Figure 4-4 are as follows:

• Scalability—Networks can grow easily because functionality is localized, so additional sites can be added easily and quickly.

• Ease of implementation—This physical topology fits easily into OSPF's logical hierarchy, making network implementation easier.

• Ease of troubleshooting—Because functionality is localized, it is easier to recognize problem locations and isolate them.

• Predictability—Because of the layered approach, the functionality of each layer is more predictable. This makes capacity planning and modeling easier.

• Protocol support—An underlying physical architecture is already in place. If you want to incorporate additional protocols, such as BGP, or if your organization acquires a network running a different protocol, you can easily add the protocol. For example, being able to connect an external network at the appropriate place in your network is enhanced with a hierarchical design.

• Manageability—The physical layout of the network lends itself to logical areas that make network management easier.

At this point, you can see that the three-layered hierarchical model fits perfectly into OSPF's logical design, and it is this model on which you will be basing your network design. Before discussing how to implement and design this type of model, some basic OSPF backbone designs are reviewed.

OSPF Backbone Design in the Hierarchical Model

The process of designing the backbone area has been previously discussed, so it is only briefly reviewed here. Always keep the backbone area as simple as possible by avoiding a complex mesh. Consider using a LAN solution for the backbone. The transit across the backbone is always one hop, latency is minimized, and the backbone is a simple design that converges quickly. Figure 4-5 illustrates a simple OSPF backbone design.

Figure 4-5 Simple OSPF Backbone Design

OSPF Backbone Area 0

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You need to keep users off the backbone because it is only a transit area. Because all other OSPF areas connect to the backbone, consider adding logical security. In OSPF, logical security can be in the form of route authentication. Furthermore, the entire network can be protected by using route authentication.

Also consider physically securing your backbone. As a network critical shared resource, the routers need to be physically secure. If you use the previously mentioned LAN backbone solution, securing your network is relatively easy; just place the backbone equipment in a secure closet or rack, as shown in Figure 4-6.

Figure 4-6 Isolate and Secure the Backbone Physically and Logically

OSPF LAN Backbone in

WAN Backbone Secure Closet

Figure 4-6 Isolate and Secure the Backbone Physically and Logically

OSPF LAN Backbone in

WAN Backbone Secure Closet

Backbone routers

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