OSPF ABR summarization - Something Wrong

Hay guys;

I am working on OSPF and I have prepared a lap to practic the summarization and redistribution. So, I have the ABR between Area 0 and Area 1. In Area 1 I have the networks 172.16.1.0 through 172.16.5.0 attached to the Internal Router R1.

My problem is when I am trying to summarize these Networks using 172.16.1.0/22 I am having the Message "Inconsistent Address/Mask 172.16.1.0/22".

I know for sure that networks from 1 to 5 will take 3 bits and then I have left 5 from that portion. Now I have already 16 bit similar plus 5 is 22. So, what is wrong?

Note: I have having a BSCI course and during the lap, the instractor got stuck in the same problem, so I wanted to figure out what is the problem and made the lap at home, but it did not work with me as well.

Any help?

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

mikem2te

abdulwart wrote: »

Hay guys;

I am working on OSPF and I have prepared a lap to practic the summarization and redistribution. So, I have the ABR between Area 0 and Area 1. In Area 1 I have the networks 172.16.1.0 through 172.16.5.0 attached to the Internal Router R1.

My problem is when I am trying to summarize these Networks using 172.16.1.0/22 I am having the Message "Inconsistent Address/Mask 172.16.1.0/22".

I know for sure that networks from 1 to 5 will take 3 bits and then I have left 5 from that portion. Now I have already 16 bit similar plus 5 is 22. So, what is wrong?

Note: I have having a BSCI course and during the lap, the instractor got stuck in the same problem, so I wanted to figure out what is the problem and made the lap at home, but it did not work with me as well.

Any help?

Your summary is wrong. Try 172.16.0.0/22

EDIT-
When you specify a summary, you need to specify the network address element of the IP address only.

For a 22 bit mask the binary representation of the mask in the third octet is 11111100, so to get the network in this case is easy, the third octet in your summary is all zeros 00000000 so performing a logical and gives a summary address of '00000000'. Hence 172.16.0.0/22.

If you try to use 172.16.1.0/22, the third octet in your summary is 00000001, as the mask is 11111100, the router actually thinks you are specifiying a host within that network rather than the network address of that network.

Hopefully that makes sense - I've probably not described that very eloquently.

mikem2te

abdulwart wrote: »

I know for sure that networks from 1 to 5 will take 3 bits and then I have left 5 from that portion. Now I have already 16 bit similar plus 5 is 22. So, what is wrong?

Your logic appears a bit flawed here.

Summarising to /22 includes 172.16.0.0 to 172.16.3.0.

abdulwart

mikem2te wrote: »

Your summary is wrong. Try 172.16.0.0/22

EDIT-
When you specify a summary, you need to specify the network address element of the IP address only..

Well, now I understand that I have to specify the network address instead of the IP address. That is great help and new for me.

However, if I have the networks which start from let us say 172.16.100.0 to the network 172.16.120.0 and I want to summarize them. Applying the same scenario, now we have 172.16 are similar in all the networks plus one bit from the third octet (the 128 bit). So, the mask will be /17.

If I am going to use the network 172.16.0.0/17 then I am wasting alot of IP addresses where other routers will think those networks (from 1 to 99) are attached to my router because I have the summary 172.16.0.0/17.

mikem2te wrote: »

For a 22 bit mask the binary representation of the mask in the third octet is 11111100, so to get the network in this case is easy, the third octet in your summary is all zeros 00000000 so performing a logical and gives a summary address of '00000000'. Hence 172.16.0.0/22.

I did not get the point in the above statment, can you illestrate more?

mikem2te

abdulwart wrote: »

Well, now I understand that I have to specify the network address instead of the IP address. That is great help and new for me.

However, if I have the networks which start from let us say 172.16.100.0 to the network 172.16.120.0 and I want to summarize them. Applying the same scenario, now we have 172.16 are similar in all the networks plus one bit from the third octet (the 128 bit). So, the mask will be /17.

If I am going to use the network 172.16.0.0/17 then I am wasting alot of IP addresses where other routers will think those networks (from 1 to 99) are attached to my router because I have the summary 172.16.0.0/17.

I did not get the point in the above statment, can you illestrate more?

That is the problem with summarisation. Good initial network design helps avoid this scenario very much but some times this is not possible.

In my opinion having 172.16.0.0/17 does waste too many networks. Maybe instead of having one large wasteful summary, use a few smaller summaries that will make up the required range such as-
172.16.100.0/22
172.16.104.0/21
172.16.112.0/21

Maybe some of the more experienced guys on this forum will have a better suggestion.

Don't worry about that statement I made, I know what I was trying to say but I didn't say it very well. I think you get what I was trying to say.

abhustler

abdulwart wrote: »

Well, now I understand that I have to specify the network address instead of the IP address. That is great help and new for me.

However, if I have the networks which start from let us say 172.16.100.0 to the network 172.16.120.0 and I want to summarize them. Applying the same scenario, now we have 172.16 are similar in all the networks plus one bit from the third octet (the 128 bit). So, the mask will be /17.

If I am going to use the network 172.16.0.0/17 then I am wasting alot of IP addresses where other routers will think those networks (from 1 to 99) are attached to my router because I have the summary 172.16.0.0/17.

I did not get the point in the above statment, can you illestrate more?

If you want to summarize 172.16.100.0 through 172.16.120.0 you can have a summary statement of 172.16.96.0 /19. This would summarize 172.16.96.0 through 172.16.127.255