Hierarchical IP Address Design and Summarization

A hierarchical IP address design means that addresses are assigned in a hierarchical manner, rather than randomly. The telephone network provides a good analogy. This network is divided into countries, which in turn are divided into areas and local exchanges. Phone numbers are assigned based on location. For example, in North America, 10-digit phone numbers represent a 3-digit area code, a 3-digit central office code, and a 4-digit line number. So if you are in Europe and you want to call someone in Canada, you dial his country code followed by his area code, central office, and line number. The telephone network switches in Europe recognize the country code and send the call to Canada; they don't have to worry about the details of the phone number. The switches in

Canada send the call to the appropriate area code, to the central office, and finally to the correct line.

This hierarchical structure allows the telephone switches to keep less detailed information about the network. For example, a central office (CO) switch only needs to know how to get to the numbers served by its equipment, and how to get to other COs and other area codes, but it doesn't need to know how to get to the specific numbers in other COs. For example, 416 is the area code for downtown Toronto. Switches outside of Toronto only need to know how to get to 416; they don't need to know how to get to each number in Toronto. Area code 416 can be considered to be a summary of Toronto.

An IP network can use a similar hierarchical structure to get comparable benefits. When routers only have summary routes instead of unnecessary details, their routing tables are smaller. Not only does this save memory in the routers, but it also means that routing updates are smaller and therefore use less bandwidth on the network. Hierarchical addressing can also result in a more efficient allocation of addresses. With some routing protocols (known as classful routing protocols), addresses can be wasted if they are assigned randomly (as explained further in the "Classifying Routing Protocols" section, later in this chapter.)

To illustrate, consider the network shown in Figure 3-5. Subnet addresses were assigned sequentially as the subnets were created, regardless of architecture, resulting in a random pattern. Consequently, when Router A sends its routing table to the other routers, it has no choice but to send all its routes.

Figure 3-5. Router A Cannot Summarize Its Routes Because of Random Address Assignment

[View full size image]

Figure 3-5. Router A Cannot Summarize Its Routes Because of Random Address Assignment

[View full size image]

Contrast this to the network in Figure 3-6, in which subnets were assigned in a hierarchical manner. Notice, for example, that all the subnets under Router A start with 10.1, while all under Router B start with 10.2. Therefore, the routers can summarize the subnets. When they communicate to other routers, they don't send all the detailed routes; they just send the summary route. Not only does this save bandwidth on the network (because smaller updates are sent), but it also means that the routing tables in the core are smaller, which eases processing requirements. It also means that small local problems don't need to be communicated network-wide. For example, if network 10.1.1.0 under Router A goes down, the summary route 10.1.0.0/16 does not change, so the routers in the core and other areas are not told about it. They do not need to process the route change, and the update does not use bandwidth on the network. (If traffic is routed to a device on that network that is down, Router A will respond with an error message, so the network can continue to function normally.)

Figure 3-6. Router A Can Summarize Its Routes, Resulting in

Smaller Routing Tables

[View full size image]

Figure 3-6. Router A Can Summarize Its Routes, Resulting in

Smaller Routing Tables

[View full size image]

The summary routes shown in Figure 3-6 are obviousall the subnets under Router A start with 10.1 and thus the summary route is 10.1.0.0/16. It isn't always this easy.

For example, consider a network in which Router A has the following subnet routes in its routing table: 192.168.3.64/28, 192.168.3.80/28, 192.168.3.96/28, and 192.168.3.112/28. Router B in the same network has the following subnet routes in its routing table: 192.168.3.0/28, 192.168.3.16/28, 192.168.3.32/28, and 192.168.3.48/28. What is the summary route for Router A's subnets? While you might be tempted to use 192.168.3.0/24 because they all have the first three octets in common, this won't work. If both Routers A and B reported the same 192.168.3.0/24 summary route, traffic would not necessarily go to the correct router, resulting in a nonfunctioning network. Instead, you have to determine the summary routes on nonoctet boundaries. Figure 3-7 illustrates how this is done.

Was this article helpful?

+2 0

Post a comment