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Community Member

OSPF : Summarization Issue !

i am trying to summarise following networks on the ABR but its giving an error.

Networks :

192.168.1.16 /28

192.168.1.32 /28

192.168.1.48 /28

192.168.1.64 /28

p1r1(config-router)#area 1 range 192.168.1.16 255.255.255.192

% OSPF: Inconsistent address/mask 192.168.1.16/26 for area range <<<<<<<<<

p1r1(config-router)#

Please suggest !

7 REPLIES
Community Member

Re: OSPF : Summarization Issue !

hi,

if you want exclusive summarization (only these networks), than you could summarize ony to these:

192.168.1.16/28

192.168.1.32/27

192.168.1.64/28

with inclusive summary (less summaries, but other networks will as well be included):

192.168.1.0/25

or

192.168.1.0/26, 192.168.1.64/28

hope this helps

anton

Community Member

Re: OSPF : Summarization Issue !

Hi,

I think in this case the correct summarisation mask would be /25 not /26 becouse the 192.168.1.64 network is not a

part of the /26 summary.

x.x.x.00010000

x.x.x.00100000

x.x.x.00110000

x.x.x.01000000

Hope this helps!

Community Member

Re: OSPF : Summarization Issue !

Dear Friends,

Fine ! i got what you wanna explain but again there is a doubt.

By norms, if i have 4 contiguous networks to be summarized, its 4 = 2 raise to power 2.

Hence, 2 bits needs to be reduced from the network part of the mask. And the subnet being defined in the configuration is the least among all. Am i right ? Please clarify.

192.168.1.16 /28

192.168.1.32 /28

192.168.1.48 /28

192.168.1.64 /28

p1r1(config-router)#area 1 range 192.168.1.16 255.255.255.192

% OSPF: Inconsistent address/mask 192.168.1.16/26 for area range <<<<<<<<<

p1r1(config-router)#

Thanks.

Re: OSPF : Summarization Issue !

Hi,

You understanding regarding subnets and summarisation is not correct.

Probably you might want to have a look at this document to clarify the concepts.

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf#search=%22everything%20you%20wanted%20to%20know%20about%20ip%20addressing%22

In your example,

192.168.1.16 /28

192.168.1.32 /28

192.168.1.48 /28

192.168.1.64 /28

You cannot summarise this in to a 192.168.1.16/26.

A /26 means, the first 2 bit in the last octet is reserved for the network.

Last octet bit wise & their decimal value

*****************************************

128 64 32 16 8 4 2 1

Only the following values are possible in the last octet, if you reserve the first 2 bits( in the last octet).

1 => 0

2 => 64

3 => 128

4 => 192

So it can only be

192.168.1.0/26

192.168.1.64/26

192.168.1.128/26

192.168.1.192/26

You can clearly see that the /28 networks that you mentioned are spanning across the first 2 /26 subnets.

Kindly spend some time reading the document that i have highlighted in the above URL. It will be very good to understand the concepts involved in IP addressing.

Hope this helps. Rate the post if you find it useful.

-VJ

Community Member

Re: OSPF : Summarization Issue !

Dear VJ,

Thanks for the detailed reply.

Let me go through the detailed document once.

Will definately revert back to you with an update.

Cheers !

Hemant.

Community Member

Re: OSPF : Summarization Issue !

Hi Hemant,

You can try ..

1.

p1r1(config-router)#area 1 range 192.168.1.0 255.255.255.192

Above line covers 1.0 to 1.63

Let this be 192.168.1.64 /28 propogated individually outside the area.

ELSE

2. you can use 192.168.1.0 /25 sunbet in area range.

In both the solutions you cover more IPs than what u have . You have no absolute best solution as the subnets u mentioned cannot be summerized in one line.

Thx,

G

Community Member

Re: OSPF : Summarization Issue !

After reading the web document that the other poster sent you, you might also want to download the Boson utility called IP Subnet Calculator from http://www.boson.com/FreeUtilities.html. It'll make your life much easier when you're trying to break up or aggregate/summarise subnets.

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