Summarization Techniques

[vishalpatil86]

Hi,

I have a question about the summarization that has been designed in attached topology.

Why can't I start addressing the area 9 with 10.64.66.0/24 (instead of 10.64.68.0/24). According to my instructor, if I used 10.64.66.0/24, the summary would overlap with other networks. Second question is how can I check if one summary overlaps with other?

Thanks

Vish

[Jon Marshall]

Vish

I think in your post you may be using the wrong subnet masks ie. you are using /24's but your diagram isn't.

So it's difficult to say whether there is an overlap because you haven't told us the correct subnet mask.

As a general answer why would you want to start with that network anyway ie. it is not in use anywhere in the diagram.

Jon

[vishalpatil86]

Its really difficult for me to explain what i want.

following is the scenario -

- Supernet is given which is 10.64.64.0/21

- Also, N1 network address is already given which is 10.64.64.0/24

Following are the summarization requirements -

- R4 sends Area 9  summary address to R3

-R3 sends Area 5 summary address to R4

- R2 sends OSPF domain summary to R1

- R1 sends eigrp summary to ospf domain

How can I design this?

I understood the topology, but I just wanna know how it has come up with this ip addressing summarization design given in the topology?

[Jon Marshall]

The diagram fits exactly with your description of the requirements.

Did you see my second post as to how to work these out ?

If you go through the addressing using that post you should be able to see why those summary ranges have been used.

If it still isn't clear can you clarify exactly where you are having problems.

Jon

[vishalpatil86]

Hi Jon

Thank you so much for the explanation

I also referred below link -

https://learningnetwork.cisco.com/thread/23983

[Jon Marshall · Accepted solution]

Forgot to answer second question.

Quick way of doing it is to take first octet in subnet mask that is not 255 and subtract that value from 256.

This will give you the number that your networks increase by -

10.64.68.0 255.255.252.0

first non 255 octet is 252 so 256 - 252 = 4.

So your networks go up in multiples of 4 using a subnet mask of 255.255.252.0  ie.

10.64.0.0
10.64.4.0
10.64.8.0
....
10.64.64.0
10.64.68.0
10.64.72.0
etc.

Jon

[vishalpatil86]

how did you come up with 10.64.68.0 255.255.252.0 ?

My apologies if i am being dumb. But, this summariztaion thing is really bugging me.

[Jon Marshall · Accepted solution]

No apologies necessary, it can be confusing.

I took that summary range from your diagram.

So in your diagram that is the range being advertised for all the OSPF areas.

The subnets in use within the OSPF areas are -

10.64.68.0 255.255.255.0
10.64.69.0 255.255.255.0
10.64.70.0 255.255.255.0
10.64.71.0 255.255.255.128
10.64.71.128 255.255.255.128

notice that all these subnets follow on from each other.

So the actual range you are trying to present is -

10.64.68.0 -> 10.64.71.255

if you go back to my other post you can see a 255.255.252.0 subnet mask covers this range and only this range.

Bear in mind I gave you the shortcut but if you are having trouble understanding this then you may want to do it all in binary until you are comfortable with it.

What I suggest you do is use my other post and work through the example topology to see if it starts to make sense.

And don't feel you can't post back because this sort of thing can be tricky until it finally clicks.

Jon

[Jon Marshall]

Vishal

Not sure whether this will help or confuse but going off your original post it's important to understand you cannot just use any starting subnet and apply a subnet mask.

So 10.64.68.0 255.255.252.0 is a valid summary range and covers what we discussed in another post.

However 10.64.66.0 255.255.252.0 is not a valid summary range ie. it does not cover -

10.64.66.0 -> 10.64.69.255

and that is because if you refer to my earlier post there you can see it is either -

10.64.64.0 255.255.252.0

or

10.64.68.0 255.255.252.0

but it can't be 10.64.66.0 255.255.252.0 because 66 is not a multiple of 4.

That is a very common mistake people make when first learning this sort of thing ie. simply picking where they want to start and going from there.

That is why Paul's example may help because he has done it in binary and it becomes more obvious when you do it that way.

Jon

[vishalpatil86]

Hi Jon,

Here is what I understood from your posts-

i summarized the eigrp domain as 10.64.64.0/23 which invloves increment of 4

Only because 10.64.66.0 and 10.64.67.0 ae included in the above network (10.64.64.0/23), i cannot really start on 10.64.66.0 cause of the overlapping. Instead I start on .68

Another question i have is cant I user those two blocks of addresses (10.64.66.0,10.64.67.0)? or I can use it with summarization disabled?

[Jon Marshall]

A /23 is a subnet mask of 255.255.254.0.

And using the calculation I gave before 256 - 254 = 2 so your networks go up in increments of 2 not 4 ie.

10.64.64.0/23 = 10.64.64.0 -> 10.64.65.255

so 10.64.66.0/24 and 10.64.67.0/24 are not included in that range and are not used anywhere in your diagram as far as I can see.

You can use those subnets if you wanted to and you could summarise them as -

10.64.66.0/23  

but you can't summarise them with a /22 mask ie. 255.255.252.0 because 66 is not a multiple of 4.

That's what I meant when I said you can't just start at any network and summarise without making sure it works out.

Jon

[Jon Marshall]

Vishal

Just to answer your last question.

You can use those blocks without summarisation yes.

Jon

[paul driver]

Hello Jon

I hope you don’t mind me joining this discussion?

Taking the 3rd octet into consideration I get the summary route to be- 10.64.64.0/21

3rd octet:
64 = 01000000
65 = 01000001
68 = 01000100
69 = 01000101
70 = 01000110
71 = 01000111

res
Paul
 

[Jon Marshall]

Hi Paul

Not at all and this is one of those topics where multiple different viewpoints can really help because the way I do it and understand it may not help others.

Jon

[JoshGarmonCiscoCCNA]

hey on this topic,

Summarization can only occur AFTER Redistribution correct?