(Thanks to David Beach, CIS Instructor at Central New Mexico Community College for this procedure.)
9 Jan 2014
kenglong@gmail.com
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Route Summarization is, in a nutshell, the compaction of many network addresses into one that summarizes them all. This summary route can then be propagated to other routers in the system. This makes for a smaller routing table and more efficient use of resources. First of all, the networks to be summarized must all be contiguous to prevent wasted address space or routing anomolies.
Let's start with the following networks that we want to summarize into one.
172.16.1.0 /24
172.16.2.0 /24
172.16.3.0 /24
172.16.4.0 /24
First, convert the above network addresses to their binary equivalents.
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 1 0. 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 1 1 . 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 1 0 0 . 0 0 0 0 0 0 0 0
Starting with the left-most bit, work your way to the right, counting all the bits that are the same across all four network addresses. In this case, 21 bits are the same. (HINT: A shortcut method is to work only in the octet that changes. We know the first octet makes 8, adding the second makes 16, and we have 5 bits that are the same in the third octet making 21.)
- - - - - - - - - - - - - - - - - - - - - - | (21 bits are the same)
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 1 0 . 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 0 1 1 . 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 . 0 0 0 1 0 0 0 0 . 0 0 0 0 0 1 0 0 . 0 0 0 0 0 0 0 0
- - - - - - - - - - - - - - - - - - - - - - |
Now we have all the information to summarize these four networks. Using the lowest network number and the number of same bits, write out the summarization like this:
172.16.1.0 /21
That is the network you will advertise. The way this works is this router will advertise only the summary route, saying "any traffic for this block of networks, send to me. I know what to do with it."
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