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230 Chapter 7: Advanced Routing Protocol Topics

Route Summarization Strategies

As mentioned earlier, route summarization works best when the network engineer plans his choice of subnet numbers anticipating route summarization. For instance, the earlier examples assumed a well-thought-out plan. The only place where subnets beginning with 10.2 were added was at the Yosemite router. That convention allowed for the creation of a summary route for all addresses beginning with 10.2 by having Yosemite advertise a route describing subnet 10.2.0.0, mask 255.255.0.0.

Some summarized routes combine many routes into one route, but that might not be the “best” summarization. “Best” generally implies, in this case, that the summary should include all the subnets specified in the question and a few other IP addresses that do not exist.

For instance, in the earlier summarization example, four subnets (10.2.1.0, 10.2.2.0, 10.2.3.0, and 10.2.4.0, all with mask 255.255.255.0) were summarized into 10.2.0.0, all with mask 255.255.0.0. However, this summary includes a lot of IP addresses that are not in those four subnets. Does the summary work given that network’s design goals? Sure. However, instead of just defining a summary that encompasses all routes, you might want to list the “tightest,” “most concise,” or “best” summary—the summary that includes all the subnets but as few extra subnets (the ones that have not been assigned yet) as possible. This section describes a strategy for finding those concise summaries.

The following list describes a generalized process by which you can summarize a group of subnets into one summary route:

Step 1 Find the parts of the subnet numbers that are identical, moving left to right. (For our purposes, consider this first part the “in common” part.)

Step 2 The summary route’s subnet number has the same value in the “in common” part of the summarized subnets and binary 0s in the second part.

Step 3 The subnet mask for the summary route has binary 1s in the “in common” part and binary 0s in the rest of the mask.

Step 4 Check your work by calculating the range of valid IP addresses implied by the new summary route, comparing the range to the summarized subnets. The new summary should encompass all IP addresses in the summarized subnets.

For instance, the earlier example used 10.2 to begin all subnets off Yosemite. Therefore, the “in common” part is the first two octets. By examining the network diagram, and knowing the convention of placing only subnets beginning with 10.2 off Yosemite, you know that there are no subnets beginning with 10.2 somewhere else in the network. In Step 2, because the “in common” part is the first two octets, the subnet number is 10.2.0.0. In Step 3, again, because the “in common” part is the first two octets, the mask is 255.255.0.0—two octets of binary 1s and two octets of binary 0s.

Route Summarization and Variable-Length Subnet Masks 231

Although this simple example might be interesting, the process (so far) still has two problems:

Most summaries force you to notice “in common” parts of subnets that do not use octet boundaries, so you have to look at the numbers in binary.

To find the “best,” “most concise,” or “tightest” summary, you almost always need to use summaries that use a more difficult mask.

By looking at the subnet numbers in binary, you will more easily discover the bits in common among all the subnet numbers. By using the longest number of bits “in common,” you will find the best summary.

Sample “Best” Summary on Seville

Seville has subnets 10.3.4.0, 10.3.5.0, 10.3.6.0, and 10.3.7.0, all with mask 255.255.255.0. You start the process by writing down all the subnet numbers in binary:

0000 1010 0000 0011 0000 01 00 0000 0000 - 10.3.4.0 0000 1010 0000 0011 0000 01 01 0000 0000 - 10.3.5.0 0000 1010 0000 0011 0000 01 10 0000 0000 - 10.3.6.0 0000 1010 0000 0011 0000 01 11 0000 0000 - 10.3.7.0

Even before looking at the numbers in binary, you can guess that the first two octets are identical in all four subnets. So, at least the first 16 bits, going from left to right, are identical among all four subnets. This means that the “in common” part is at least 16 bits long. Further examination shows that the first 6 bits of the third octet are also identical, but the seventh bit in the third octet has some different values. So the “in common” part of these four subnets is the first 22 bits.

Step 2 says to create a subnet number for the summary by taking the same value for the “in common” part and binary 0s for the rest. In this case:

0000 1010 0000 0011 0000 01 00 0000 0000 - 10.3.4.0

Step 3 creates the mask by using binary 1s for the “in common” part and binary 0s for the rest. The “in common” part in this example is the first 22 bits:

1111 1111 1111 1111 1111 11 00 0000 0000 - 255.255.252.0

So, the summary route uses subnet 10.3.4.0, mask 255.255.252.0.

Step 4 suggests a method to check your work. The summary route should include all the IP addresses in the summarized routes. In this case, the range of addresses starts with 10.3.4.0. The first valid IP address is 10.3.4.1, the final valid IP address is 10.3.7.254, and the broadcast address is 10.3.7.255. In this case, the summary route includes all the IP addresses in the four routes it summarizes and no extraneous IP addresses.