Router summarization question

dayglow

Hi,

I'm currently studying for my CCNA and I'm struggling with router summarization.
Have look at this example:



In my book they say that you can summarize addresses when the number of addresses is a power of 2.
I know how summarization works but I don't understand why the addresses from the router in the upper left corner can be summarized. There are 3 subnets and to my knowledge, 3 is not a power of 2. So, why can they be summarized? And if they can be summarized, why can't the subnets from the router in the bottom left corner be summarized?

Thanks in advance for your help

james43026

I think this video will help you understand how to calculate summary routes. Just forget what you've learned thus far, as it sounds like you may have learned some confusing material. As far as the diagram goes, it shows that all three routers on the left, could have all of it's routes summarized, and then lists the summary addresses that the routers would advertise directly to the right of them. The bottom one may look confusing because it's using two summary routes. This is because the networks on the bottom left router aren't contiguous, and cannot be placed into one summary address without possibly causing routing loops.

Mooseboost

james43026 wrote: »

aren't contiguous, and cannot be placed into one summary address without possibly causing routing loops.

Always keep this in mind when summarizing addresses. For the CCNA exam, be very careful with this on exam day.

dayglow

Thanks for the replies.
I understand what's explained in the video but I'm still having difficulties with the words contiguous and discontiguous. Is there an easy way to see if networks are contiguous (or discontiguous)?

james43026

A contiguous set of networks would mean that they are in sequential order. So 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24, 192.168.4.0/24, are all contiguous, because they are in sequential order. Discontiguous is the opposite, meaning they are not in sequential order. So 192.168.1.0/24, 192.168.3.0/24, 192.168.5.0/24, 10.0.0.0/8, none of these networks are in sequential order so they are discontiguous.

volfkhat

dayglow wrote: »

Thanks for the replies.
I understand what's explained in the video but I'm still having difficulties with the words contiguous and discontiguous. Is there an easy way to see if networks are contiguous (or discontiguous)?

Personally,
i recommend that you cease using the term "networks". In my opinion, it's somewhat too vague when it comes to learning this stuff.

Instead, use the term "Subnet".

So, you want some examples of contiguous subnets.

Here's an example :
(assume a tradational Subnet mask of 255.255.255.0)
192.168.1.0
192.168.2.0
192.168.3.0
192.168.4.0

There you have it.
I just listed 4 contiguous subnets.
Just by 'eyeballing' it... i think you can see WHY they are contiguous.


So how about a discontiguous example:
(again, assume a 255.255.255.0 sub-mask)
192.168.1.0
192.168.2.0
192.168.4.0
192.168.5.0

Do you see the difference?
In this example, there is block of ip addresses that is missing; the 192.168.3.0 subnet.

Thus,
these 4 subnets are NOT contiguous.


I hope that my example made sense.

dayglow

Thanks for the explanation. In the examples you gave it is indeed easy to see if subnets are contiguous or not. But how about the 3 subnets (172.16.1.128/26, 172.16.1.192/26 and 172.16.2.0/24) from the bottom left router. Why are they discontiguous? Here's my reasoning:

subnet 172.16.1.128/26 ranges from 172.16.1.128 to 172.16.1.191 and has an increment of 64 in the 4th octet
subnet 172.16.1.192/26 ranges from 172.16.1.192 to 172.16.1.255 and has an increment of 64 in the 4th octet
subnet 172.16.1.2.0/27 ranges from 172.16.2.0 to 172.16.2.255 and has an increment of 1 in the 3rd octet

I don't see any "skipped" subnet. So, I don't understand why they are discontiguous. I am surely missing something but I don't know what.

volfkhat

dayglow wrote: »

Thanks for the explanation. In the examples you gave it is indeed easy to see if subnets are contiguous or not. But how about the 3 subnets (172.16.1.128/26, 172.16.1.192/26 and 172.16.2.0/24) from the bottom left router. Why are they discontiguous? Here's my reasoning:

subnet 172.16.1.128/26 ranges from 172.16.1.128 to 172.16.1.191 and has an increment of 64 in the 4th octet
subnet 172.16.1.192/26 ranges from 172.16.1.192 to 172.16.1.255 and has an increment of 64 in the 4th octet
subnet 172.16.1.2.0/27 ranges from 172.16.2.0 to 172.16.2.255 and has an increment of 1 in the 3rd octet

I don't see any "skipped" subnet. So, I don't understand why they are discontiguous. I am surely missing something but I don't know what.

Hmmm... i see what you mean.

And, in fact, i agree with your logic.
I would argue that the 3 subnets ARE contiguous.

Additionally, if you asked me to SUMMARIZE them under a single route....
the summarization would be

# ip route 172.16.0.0 255.255.252.0 serial 1/0

(i made up "serial 1/0" just to use as an example)

Of course, this Summarization would actually include all addresses from
172.16.0.0 - 172.16.3.255

(after some reflective thought):
Hmmm...
so my summarization is Overly-Broad.
It is NOT an exact fit.

So perhaps that is why your material says it is Not contiguous?

Whatever.
Semantics.

It doesn't matter, imo.
What matters is that you understand HOW to summarize them together :]

I say you can summarize them, and you would do it with a --> 172.16.0.0/22

But what do i know! Perhaps someone else can point out my error?


For what it's worth, this level of detail was NOT on the ccent :]

dayglow

My summary route for the 3 subnets would indeed be 172.16.0.0/22

volfkhat wrote: »

Hmmm... i see what you mean.

And, in fact, i agree with your logic.
I would argue that the 3 subnets ARE contiguous.

Additionally, if you asked me to SUMMARIZE them under a single route....
the summarization would be

(i made up "serial 1/0" just to use as an example)

Of course, this Summarization would actually include all addresses from
172.16.0.0 - 172.16.3.255

(after some reflective thought):
Hmmm...
so my summarization is Overly-Broad.
It is NOT an exact fit.

So perhaps that is why your material says it is Not contiguous?

Whatever.
Semantics.

It doesn't matter, imo.
What matters is that you understand HOW to summarize them together :]

I say you can summarize them, and you would do it with a --> 172.16.0.0/22

But what do i know! Perhaps someone else can point out my error?


For what it's worth, this level of detail was NOT on the ccent :]

james43026

I wasn't exactly clear when I said they were discontiguous. And I used the incorrect term. While they are contiguous, the networks in the bottom left corner must be summarized the way they are because you can't summarize those routes into 172.16.0.0/22, what happens when anything destined for the 172.16.0.0 or 172.16.3.0 networks are sent to that router? Or even anything from 172.16.1.0 - 172.16.1.127. Either a routing loop will occur, or a black hole will occur. Neither of which you want anything to do with, as they can be a nightmare to track down and correct. This may not be a big deal if you aren't using those subnets anywhere else in your network, but as soon as you do, you now have issues. Or you simply waste those IP addresses. This is one of the biggest if not the biggest rule in route summarization. The example is made all the more difficult by the fact that VLSM has been used.

Hopefully this clears up any confusion on this.

dayglow

Thanks for clarifying this.

james43026 wrote: »

I wasn't exactly clear when I said they were discontiguous. And I used the incorrect term. While they are contiguous, the networks in the bottom left corner must be summarized the way they are because you can't summarize those routes into 172.16.0.0/22, what happens when anything destined for the 172.16.0.0 or 172.16.3.0 networks are sent to that router? Or even anything from 172.16.1.0 - 172.16.1.127. Either a routing loop will occur, or a black hole will occur. Neither of which you want anything to do with, as they can be a nightmare to track down and correct. This may not be a big deal if you aren't using those subnets anywhere else in your network, but as soon as you do, you now have issues. Or you simply waste those IP addresses. This is one of the biggest if not the biggest rule in route summarization. The example is made all the more difficult by the fact that VLSM has been used.

Hopefully this clears up any confusion on this.