Route summarization

I thought I had route summarization figured out a while ago, but now i seem to be missing a big part of it.
Given the addresses:

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

I line up the smallest and the largest and get a summary of 192.168.0.0/21
but the correct answer gives me 2 summaries.
Why? These addresses are contiguous.

any help is appreciated!

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Comments

AwesomeGarrett

What is the question? Does it ask for a single summary? Yes, your answer would be correct if it was explicitly asking for a single summary subnet.

theodoxa

192.168.0.0/21 would cover 192.168.0.x - 192.168.7.x. If it is not allowable to use a summary that includes addresses outside the ranges given, then you would have to use 192.168.0.0/22 and 192.168.4.0/23. These two summaries would cover the networks listed and no others.

agtjamesb007

theodoxa seems to be on track towards what they are looking for. The exact question regarding multi area ospf reads: router has six networks that are connected to it with network IDs of 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, and 192.168.5.0/24. After summarization of these six networks, how many LSAs would be sent to advertise the summary route or routes?
The answer is two
The summary routes for the six networks would be 192.168.0.0/22 and 192.168.4.0/23. One LSA is needed to advertise each summary route.

But I am confused as to how you come to summarize to those two routes. How do you know what addresses are included in a given summary?

AwesomeGarrett

What are your thoughts? If you don't know, I would suggest to review your subnetting notes and come back with an answer.

This is basic concept and I dont just want to give you the answer.

agtjamesb007

I have the answer, Its the explanation I need. I cant learn without an explanation. That's why I came to the forum.

OfWolfAndMan

You're not wrong in your sub netting, and I think I know where you're getting confused. In the fact that they want two summary routes I assume? That is a strange question. If it's asking for proper sub netting, then the two would be correct, however you can't summarize two ranges for inter-area OSPF that I know of. Usually you would do:

#router ospf 1
#area 1 range 192.168.0.0 255.255.248.0
#redistribute static subnets route-map [Route filter name]
#ip route 192.168.0.0 255.255.248.0 null0 <--- Used as a bit bucket for anything .6.0 or above.

AwesomeGarrett

Can't summarize two ranges?

area 1 range 192.168.0.0 255.255.252.0
area 1 range 192.168.4.0 255.255.254.0

I don't follow.

agtjamesb007 wrote: »

How do you know what addresses are included in a given summary?

This is the question I did not want to answer. Or maybe you didn't ask the right question? Is your question, how does OSPF know what routes are actually reachable via the summary route?

theodoxa

Summarization is just subnetting in reverse. Instead of breaking one large network into many smaller networks, you are combining many smaller networks into one larger network.

Hondabuff

You just have to figure out what block size will cover these networks. Use the subnet method 2,4,8,16,32,64,128 etc

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

Block size of 4 doesn't cover 0-5 but 8 does with some waste. Take the Magic number 256 - 8= 248. Since your in the 3 octet, a 248 or a /21. If they want 2 summarized routes then I would say that is more of a trick question given the questions wording.

theodoxa

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

11000000.10101000.00000000.00000000
11000000.10101000.00000001.00000000
11000000.10101000.00000010.00000000
11000000.10101000.00000011.00000000
11000000.10101000.00000100.00000000
11000000.10101000.00000101.00000000

11000000.10101000.00000000.00000000

You could summarize the 6 networks to 192.168.0.0/21 (24 - 3 = 21), but the subnets with a 3rd octet ending in 110 and 111 were not given yet would be included in the summary. The solution to not include those subnets would be 2 summaries

11000000.10101000.00000000.00000000
11000000.10101000.00000001.00000000
11000000.10101000.00000010.00000000
11000000.10101000.00000011.00000000

11000000.10101000.00000000.00000000 (192.168.0.0/22)

and

11000000.10101000.00000100.00000000
11000000.10101000.00000101.00000000

11000000.10101000.00000100.00000000 (192.168.4.0/23)

Notice that for both summaries the bits added to the host portion include all combinations from all zeroes to all ones. (00, 01, 10, 11). This means that no additional [unwanted] subnets would be included in the summary.

Hondabuff

theodoxa wrote: »

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

11000000.10101000.00000000.00000000
11000000.10101000.00000001.00000000
11000000.10101000.00000010.00000000
11000000.10101000.00000011.00000000
11000000.10101000.00000100.00000000
11000000.10101000.00000101.00000000

11000000.10101000.00000000.00000000

You could summarize the 6 networks to 192.168.0.0/21 (24 - 3 = 21), but the subnets with a 3rd octet ending in 110 and 111 were not given yet would be included in the summary. The solution to not include those subnets would be 2 summaries

11000000.10101000.00000000.00000000
11000000.10101000.00000001.00000000
11000000.10101000.00000010.00000000
11000000.10101000.00000011.00000000

11000000.10101000.00000000.00000000 (192.168.0.0/22)

and

11000000.10101000.00000100.00000000
11000000.10101000.00000101.00000000

11000000.10101000.00000100.00000000 (192.168.4.0/23)

Notice that for both summaries the bits added to the host portion include all combinations from all zeroes to all ones. (00, 01, 10, 11). This means that no additional [unwanted] subnets would be included in the summary.

This is 100% correct. I would still rather have the one LSA of a /21 being advertised instead of 2 LSA's in the real world.

agtjamesb007

I agree. It is a tricky question. Thanks for the clarification.

theodoxa

Hondabuff wrote: »

This is 100% correct. I would still rather have the one LSA of a /21 being advertised instead of 2 LSA's in the real world.

I agree. I would have assigned the entire /21 (or /16 if I knew that I would never have more than 256 sites) to that site so that I could use a single summary address for the site. The extra subnets would simply be kept in reserve.