Route summarization questions

totts

Questions testing your knowledge of route summarization appear to be few and far between. Would someone mind setting me a couple of questions. If you wouldn't mind doing one where only 0s are matched in the final 'matching' octet and another where there'll be a number of 1's matching in the final matching octet. If I can get them then I'll be able to move on.
Many thanks, Totts

gabrielbtoledo

I didn't understand your request.
If you have all zeros in the last matching octet, then you won't need to to route summarization.
You only do route summarization where you see difference in between zeros and ones. In other words, if you go to last octet with all zeros, then you move to one octet before. And if they are all the same, then the default subnet mask is the mask you are looking for.

mikearama

I think I know what you need...

Given these subnets:
192.168.10.0 /24
192.168.11.0 /24
192.168.12.0 /24
192.168.13.0 /24
192.168.14.0 /24
192.168.15.0 /24
... what's the summary address / mask?

Now, slightly tougher (from our actual network):
10.22.178.0 /23
10.22.180.0 /23
10.22.182.0 /23
10.22.184.0 /23
10.22.186.0 /23
10.22.188.0 /23
10.22.190.0 /23
... what's the summary address / mask?

Have fun,
mike

r_durant

Can anyone answer or is this for totts only?

mikearama

Sure, anyone can have a go... but totts, no peaking at any answers until you've worked them out for yourself!!

bighornsheep

mikearama wrote:

I think I know what you need...

Given these subnets:
192.168.10.0 /24
192.168.11.0 /24
192.168.12.0 /24
192.168.13.0 /24
192.168.14.0 /24
192.168.15.0 /24
... what's the summary address / mask?

mike

I think it's 192.168.8.0 / 21

mikearama wrote:

I think I know what you need...

Now, slightly tougher (from our actual network):
10.22.178.0 /23
10.22.180.0 /23
10.22.182.0 /23
10.22.184.0 /23
10.22.186.0 /23
10.22.188.0 /23
10.22.190.0 /23
... what's the summary address / mask?

Have fun,
mike

I think it's 10.22.176.0 / 20

r_durant

Those were my answers as well bighorn...

192.168.8.0/21...encapsulates 8.0 - 7.255

10.22.176.0/20...encaps 176.0 - 191.255

bighornsheep

r_durant wrote:

Those were my answers as well bighorn...

192.168.8.0/21...encapsulates 8.0 - 7.255

10.22.176.0/20...encaps 176.0 - 191.255

192.168.8.0/21 summarizes 8.0 - 15.255

I wouldn't call blocks of network addresses an encapsulation.

r_durant

Typo on the 7.255...

Not encapsulation in the sense of framing, packets, etc...but that the subnets specified are within that range/block...was just used loosely!!

mikearama

Nicely done, guys.

Here's one more:

172.16.64.0 /20
172.16.80.0 /20
172.16.96.0 /20
172.16.112.0 /20
172.16.128.0 /20
172.16.144.0 /20
172.16.160.0 /20

Okay, so if you were the admin, and each of these represented a remote network, how would you go about summarizing your routes?

Mike

bighornsheep

mikearama wrote:

Here's one more:

172.16.64.0 /20
172.16.80.0 /20
172.16.96.0 /20
172.16.112.0 /20
172.16.128.0 /20
172.16.144.0 /20
172.16.160.0 /20

Lots of extra addresses, but this is the closest one I believe. 172.16.0.0 / 16

mikearama

And that would certainly work, bighorn... but what if 172.16.0.0 /20 and 172.16.16.0 /20 are local subnets, along with 172.16.208.0 /20, and after.. What could you do then?

bighornsheep

I just remembered you said they were remote networks.

This is a long shot, but I would say that the routes would have to go through the default gateway running NAT, since the addresses overlap with the local subnet. If I am wrong, can you explain what could be done in such a situation?

Thanks in advance.

mikearama

Those networks are remote, but that doesn't mean the entire 172.16.0.0 /16 is remote.

This kinda scheme just takes a bit of planning, since the initial ip scheme wasn't well planned. If it was, then local and remote networks would be in completely different core subnets.

You can group most of the subnets with 172.16.64.0 /18, which would cover 172.16.64.0 through 172.16.127.254

Now you have a choice... either leave the other three lines in as they are, or summarize again. Personally, I'd summarize again with 172.16.128.0 /18, which covers 172.16.128.0 through 172.16.191.254

I just wanted to get you techies thinking beyond the typical, easy, everyday subnetting question where everything fits into one summary address.

HTH,
Mike

dtlokee

bighornsheep wrote:

I just remembered you said they were remote networks.

This is a long shot, but I would say that the routes would have to go through the default gateway running NAT, since the addresses overlap with the local subnet. If I am wrong, can you explain what could be done in such a situation?

Thanks in advance.

Also keep in mind the router will attempt to make a longest match, so it would refer the 172.16.20.0/24 route over the 172.16.0.0/16 route for a packet destined to 172.16.20.100 because it is a better match. This allows you to have summary routes that overlap other routes that are locally known because it will prefer the longer match.

Tricon7

Just a question - I solved the summarization problems, but only by drawing out the bits, which is time-consuming. Is there a shortcut to keep from having to "binary out" each IP octet?

r_durant

dtlokee wrote:

Also keep in mind the router will attempt to make a longest match, so it would refer the 172.16.20.0/24 route over the 172.16.0.0/16 route for a packet destined to 172.16.20.100 because it is a better match. This allows you to have summary routes that overlap other routes that are locally known because it will prefer the longer match.

DT, when you say "longest match", do you mean that it will try to match more octects in the subnet mask versus the octects the packet is destined to?

mobri09

Tricon7 wrote:

Just a question - I solved the summarization problems, but only by drawing out the bits, which is time-consuming. Is there a shortcut to keep from having to "binary out" each IP octet?

BUMP that... any cool and quick shortcuts

dtlokee

r_durant wrote:

dtlokee wrote:

Also keep in mind the router will attempt to make a longest match, so it would refer the 172.16.20.0/24 route over the 172.16.0.0/16 route for a packet destined to 172.16.20.100 because it is a better match. This allows you to have summary routes that overlap other routes that are locally known because it will prefer the longer match.

DT, when you say "longest match", do you mean that it will try to match more octects in the subnet mask versus the octects the packet is destined to?

If you are using classless routing the router will attempt to use the entry in the routing table that matches the most bits in the packet being routed. This match depends on the subnet mask associated with the route, that is at a minimum the packet being matched must match all of the "network" (and subnet) bits designated in the mask. For example fo a packet to match 172.16.20.0/22, it must match the first 22 bits and the last 12 can be anything, Now if there was also a 172.16.21.0/24 the packet must match the first 24 bits exactally and the last 8 can be anything.

r_durant

Gotcha...thanks!!

totts

Well done mikearama for figuring out what I meant in the first place. bet that got you scratcing your head

Too late for me to be figuring that out now, I'll save it for my bacon butty in the morning,
Cheers!

bighornsheep

mikearama wrote:

You can group most of the subnets with 172.16.64.0 /18, which would cover 172.16.64.0 through 172.16.127.254

Now you have a choice... either leave the other three lines in as they are, or summarize again. Personally, I'd summarize again with 172.16.128.0 /18, which covers 172.16.128.0 through 172.16.191.254

This is a very good tip.....quite definitely a "real world scenario", Thank you for the explanation!

larkspur

any more questions? let's keep it going.

cdodson

After graduating from a Cisco Network Academy 4 years ago on the CCNA level, I am now taking the CCNP curriculum at a 4-year institution. I had forgotten what summarization was in our EIGRP module and this thread took me back and reminded me that it isn't all that complicated after all.

Dubuku57

Like mobri09 and Tricon 7 asked, are there any SHORTCUTS to figuring out the summary mask other than writing out all the bits?

I read the following in a CCNA Study Guide:

- when summarizing you can summarize routes only on a bit boundary(power of 2) or a multiple of a power-of-2 boundary
- you can create a summary route when the total number of addresses is a power of 2. If not, you'll have to divide the addresses into smaller groups and summarize them seperately.

I THINK its a shortcut? Dont understand it though, could anyone pls explain this?

kryolla

The key word is bit boundary

bakes00

mikearama wrote: »

I think I know what you need...

Given these subnets:
192.168.10.0 /24
192.168.11.0 /24
192.168.12.0 /24
192.168.13.0 /24
192.168.14.0 /24
192.168.15.0 /24
... what's the summary address / mask?
mike

Hi guys,

Please help me, I seem to have a fundamental misunderstanding here.

Everytime I do the route summary for these addresses I come up with:
192.168.0.0/20

How do you arrive at 192.168.8.0/21?

192.168.15.0 = 192.168.00001111.00000000

Only the last 4 bits are ever changed to reach 15, so the first 4 are all common among the given network addresses?


I guess I'm missing the point, but I thought I understood it.

Hope someone has the patience to help me understand how everyone arrived at the other answer.

trackit

bakes00 wrote: »

Hi guys,

Please help me, I seem to have a fundamental misunderstanding here.

Everytime I do the route summary for these addresses I come up with:
192.168.0.0/20

How do you arrive at 192.168.8.0/21?

192.168.15.0 = 192.168.00001111.00000000

Only the last 4 bits are ever changed to reach 15, so the first 4 are all common among the given network addresses?


I guess I'm missing the point, but I thought I understood it.

Hope someone has the patience to help me understand how everyone arrived at the other answer.

192.168.00001010.0
192.168.00001011.0
192.168.00001100.0
192.168.00001101.0
192.168.00001110.0
192.168.00001111.0

the last 3 bits in the third octet are different so its block of 8 (from 192.168.8.0 - 192.168.15.0). to write it in CIDR notation you get 192.168.8.0/21

bakes00

Oh no, how embarrassing, I did not read the question properly. I was going from 192.168.1.0 to 192.168.15.0 :-s

Thanks guys... i need to do tons more revision on this.

bakes00

Hi guys,

Ok, I have been practicing summarizing, but only in the forward sense.

I found an example question that reads:

- If a routers has networks connected to it that are summarized as 192.168.176.0/21, which packets will be sent to this router.

The question then lists 5 IP address options to select from.

Can anyone please show me how to do this process in the reverse to find the answer to the question?

Thanks in advance.

dynamik

That's actually just a basic subnetting question since all you have to do is find the address range for that subnet and see if the IPs fall into it or not. There's no reason to break the networks back out of it.