question on summarization

[Lassek]

Hi,

I thought I understood summarization pretty well but I found an example on the Todd Lammle book which is confusing me and makes me think that I might be missing something.

1. This first exercise asked to summarize 192.168.1.0/24 through 192.168.12.0/24

    My summarization was 192.168.0.0/20 which matches the result on the book. So far so good.

2. Another exercise asked to summarize 172.16.1.0 through 172.16.7.0

    My summarization here doesn't match the result on the book.

    I summarized it as 172.16.0.0/21 however the book summarized it as 172.16.1.0/21

    I thought the address always had to be the first one in the block, in this case a block size of 8 starting from 0. This is what I did in exercise 1 which was correct and looks very similar to the second one.

Any thoughts?

Regards

Lass

[Jon Marshall]

Lass

The book is wrong and you are right.

Edit - note that both summary addresses include more networks than the actual range you have been asked to summarise.

Jon

[Lassek]

Thanks Jon for your response, however, after you included the edits not sure what the answer is.

I can see that both summaries advertise more networks than asked:

1. 192.168.1.0/24 through 192.168.12.0/24

    Summary 192.168.0.0/20

   Advertises from 192.168.0.0 to 192.168.15.0 which is 4 more than asked, I believe this is unavoidable?

2. 172.16.1.0 through 172.16.7.0

    My summary is 172.16.0.0/21

   Advertises from 172.16.0.0 to 172.16.7.0 which is 1 more than asked, I believe this is also unavoidable?

Just to double check:

What’s the summary of 172.16.1.0 through 172.16.7.0?

a. 172.16.0.0/21 or

b. 172.16.1.0/21

Regards,

Lass

[Jon Marshall]

Lass

My sincere apologies.

I will leave that last edit to display my general stupidity to all :-)

I have totally managed to confuse you when a little bit more care with my answer would not and it would appear you have a much better understanding of summarisation than me.

You are absolutely correct, my second edit was totally wrong (deleted to avoid confusion).

So regarding your original question you are correct and the book is wrong.

Regarding my second edit you are correct and I am wrong.

Jon

[Lassek]

Thanks Jon for your help with this :)

[Jon Marshall]

No problem.

Probably is a good idea to delete it.

Jon

[Lassek]

Hi Jon

An afterthought regarding your second edit.

Thinking about next people reading this post I'd be a good idea to delete/edit it if you know it is wrong to avoid confusion

Regards

Lass

[Richard Burts]

Lass

Note that both 172.16.0.0/21 and 172.16.1.0/21 produce the same result. If you enter the command to summarize using 172.16.1.0/21 you will not get a syntax error. But if you go back and look at the running config it will show up as 172.16.0.0/21.

So your understanding is correct and the book is somewhat misleading.

HTH

Rick

[Joseph W. Doherty]

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Posting

The "book" may be incorrect on both answers but it might be correct too (although, I agree with Rick, the book's answers are misleading).

Both of the book's summary routes include more than the indicated address blocks, as also noted by Jon.  However, if you "know" a router "owns" the summary block, it's not unknown to advertize a larger block even if not all of it is actually allocated.

To extend what Rick has also mentioned, you can actually use any address within the address block to summarize.  Basically, the host portion will be ignored.  Again, though, doing this, can be misleading or confusing.

To summarize just 192.168.1.0/24 through 192.168.12.0/24, you need:

192.168.1.0/24

192.168.2.0/23

192.168.4.0/22

192.168.8.0/22

192.168.12.0/24

To summarize just 172.16.1.0(/24?) through 172.16.7.0(/24?), you need:

172.16.1.0/24

172.16.2.0/23

172.16.4.0/22

[Richard Burts]

Joseph makes a good point and reminds us that there are multiple ways to look at this question and the different orientations may produce different results about what is the "correct" answer. In the entry level certifications (and I believe in the book being referenced - though we do not know quite which book) one important concept is to find the bit boundary that produces a block of addresses that contain the given addresses. From that perspective the answers we were discussing could be considered the "correct" answer. In more advanced certifications (especially in the CCIE) we need to be very careful about exactly what the question specifies. And Joseph is quite correct that if the instruction is to summarize exactly 192.168.1.0 through 192.168.12.0 that you need the more complicated and precise set of summarizations.

HTH

Rick

[Lassek]

Thanks Joseph

It's not clear to me what you meant in the last part of your comments and what the implications are.

First let me clarify that I don't use the bit boundary in my technique to summarize, I just do it all in my head following a simple technique. However, I've read about it and I think I can do it that way too but takes longer. So what I’m trying to clarify is whether my technique is valid for all cases or there might be situations were necessarily I have to use the bit boundary. That’s why I’m trying to fully understand what you meant just in case I missed something important.

The bit which is confusing for me is:

"...To summarize just 192.168.1.0/24 through 192.168.12.0/24, you need:

192.168.1.0/24

192.168.2.0/23

192.168.4.0/22

192.168.8.0/22

192.168.12.0/24"

I can see that on the first address I need 24 bits to obtain 192.168.1.0,

for the second one I need 23 bits, 22 bits for the third. However, I don’t understand 192.168.8.0/22. Shouldn't it be 192.168.8.0/21 instead? I don’t understand the last one either.

It’s not clear to me what you meant by "you need". Let me explain.

My understating is that you can only summarize in blocks, and all address in that block will be advertised.

For instance If need to summarize x.x.1.x though x.x.7.x the best summary I can use is x.x.0.x /21 which will advertise from x.x.0.x through to x.x.7.x (8 networks)

Howevr it is not possible to just advertise a couple of the address included in the above block.

is this correct?

Regards,

Lass

[Jon Marshall]

Lass

However, I don’t understand 192.168.8.0/22. Shouldn't it be 192.168.8.0/21 instead?

Joseph was showing you how you would summarise only subnets 192.168.1.0 -> 192.168.12.0/24

You can't use 192.168.8.0/21 because this is 192.168.8.0 -> 192.168.15.255 which is going beyond 192.168.12.0/24.

So 192.168.8.0/22 is 192.168.8.0 -> 192.168.11.255 and then you have the final 192.168.12.0/24.

What was meant by need is if you only want to advertise the specific blocks ie. 192.168.1.0 -> 192.168.12.0 and no others.

All Joseph was explaining is that if you use 192.168.0.0/20 you are advertising more than 192.168.1.0/24 to 192.168.12.0/24.

That may be okay but if you only wanted to advertise out those specific subnets you can't do that with one summary address, you need to break it down.

So you can still use summary addresses as Joseph did but you need multiple ones and sometimes because of the bit boundaries you also need to specify the specific subnets as well to get to the next bit boundary.

Jon

[Lassek]

thanks Jon,

Your comments match exactly my understanding of summarization, so all good.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Perhaps your confusion lies in how summaries should be used.

Hopefully, the following will (indirectly) answer your questions.

Suppose we have four /24s, back to back, such as:

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

All four of those blocks are "covered" by 192.168.0.0/22, which we could use to summarize them.

But suppose we had:

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

192.168.4.0/24

If we use 192.168.0.0/21, that actually covers:

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

192.168.4.0/24

192.168.5.0/24

192.168.6.0/24

192.168.7.0/24

But is that a problem?

Well it might be or might not be, it depends on what else is "happening" on the network.

Further suppose, while you have 192.168.(1..4).0/24 I have 192.168.0.0/24 and 192.168.(5..7).0/24.  How do I summarize my /24s?  What if I too used 192.168.0.0/21?  If I did, we have conflicting summaries!

So, to avoid the conflict, we could summarize your 192.168.(1..4).0/24 as:

192.168.1.0/24

192.168.2.0/23 (covers 2..3)

192.168.4.0/24

Basically, the above only summarizes two of your /24s, but the above also insures we don't summarize addresses we're not using or don't own.

Of course, if we paid attention to bit boundaries, ideally your four /24s should all be covered by a /22, as would be for 192.168.0.0/22.

[Lassek]

Thanks Jon for taking the time to explain it

What you've explain matches my understanding, everything is clear.

Thanks