As your organization grows, the network must be able to keep pace. Your network and its initial design must enable the organization to expand accordingly. Therefore, you must plan into a network the capability to further expand it. A network that cannot keep pace with the organization's business needs and growth is of little use.

For example, routing summarization is a major factor in the success of designing your network. To ensure that your network can scale properly, route summarization is the biggest factor against which to measure your success. Without summarization, you have a flat address design with specific route information for every subnet being transmitted across the network—a bad thing in large networks. In discussing summarization, remember that routers summarize at several levels, as shown in Figure 4-1.

Figure 4-1 Route Summarization Affects Network Scalability

Network Routes Within Autonomous System

Subnet Routes Within Network

Host Routes

Within Subnet




Internal OSPF

Area Border





Summarized Network Advertised

166 Chapter 4: Design Fundamentals

For example, hosts are grouped into subnetworks, subnetworks are then grouped into major networks, and major networks are then consolidated into OSPF areas. The overall OSPF network can then be grouped into an autonomous system (AS). Chapter 7, "Summarization," covers this topic in more detail.

NOTE Many smaller networks can use a standard routing protocol such as OSPF. For example, these networks can have 100 or less routers with a relatively small IP space. In these situations, summarization might not be possible and might not gain much if it were implemented.

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