Summarization doubt

Answered Question
Jul 6th, 2014
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Hi Experts,

 

I want to understand summarization.  As per my understanding. summarization is based upon no of "1 bits" but doesnot subnet mask matters for summarization ?

 

e.g. 10.1.1.1/8

10.1.1.2/8

10.1.1.3/8

10.1.1.4/8

 

10.1.1.1/8

10.1.1.2/9

10.1.1.3/10

10.1.1.4/11

 

I want to understand whether summarization depends upon submetmask or not?

 

 

 

Correct Answer by petenixon about 3 years 1 month ago

To calculate a summary route, find the lowest address then the highest and then (the most important bit) recalculate using the required block size.

If you're using your first example, you wouldn't use a summary route as you're using the entire class A network range but if you wanted, you could use 10.0.0.0/8 as a summary. If we were to change that to a /24 we could end up with some networks like this:

10.1.1.0/24
10.1.2.0/24
10.1.3.0/24
10.1.4.0/24

First address = 10.1.1.0
Last address = 10.1.4.255

Now find a block size that will encompass those address ranges. Working from the third octet as the first two will not change:

10.1.0.0/21 will be our summary route using a block size of 8. 4 will be too small as this won't encompass the .4 addresses.

Don't forget that when recalculating using your block size, that you start at 0 (first subnet).

There are a lot more eloquent (and probably easier) explanations on the Cisco learning forum, but this is an easy way for me to remember and I hope this helps.

In short however, only worry about the subnet mask when finding your first and last addresses that you wish to summarize.


 

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Correct Answer
petenixon Mon, 07/07/2014 - 04:09
User Badges:

To calculate a summary route, find the lowest address then the highest and then (the most important bit) recalculate using the required block size.

If you're using your first example, you wouldn't use a summary route as you're using the entire class A network range but if you wanted, you could use 10.0.0.0/8 as a summary. If we were to change that to a /24 we could end up with some networks like this:

10.1.1.0/24
10.1.2.0/24
10.1.3.0/24
10.1.4.0/24

First address = 10.1.1.0
Last address = 10.1.4.255

Now find a block size that will encompass those address ranges. Working from the third octet as the first two will not change:

10.1.0.0/21 will be our summary route using a block size of 8. 4 will be too small as this won't encompass the .4 addresses.

Don't forget that when recalculating using your block size, that you start at 0 (first subnet).

There are a lot more eloquent (and probably easier) explanations on the Cisco learning forum, but this is an easy way for me to remember and I hope this helps.

In short however, only worry about the subnet mask when finding your first and last addresses that you wish to summarize.


 

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