IP Summarization

kwyap · August 2006

Hi,

Please advise, I have netowrk
172.16.0.0/16
172.17.0.0/16
172.18.0.0/16
172.19.0.0/16

Could these network summarized as 172.16.0.0/14 ?
I'm going to do a manual summarization on RIPv2 interface.

TQVM in advance.

JVE · August 2006

well 172. is a class B, so really i dont think you will get less then 16 bits with that. You need to work with that third octet

mikej412 · August 2006

kwyap wrote:

Could these network summarized as 172.16.0.0/14 ?

Plugging that Network Address and Prefix into the TechExams.net IP Subnet Calculator, I get a mask of 255.252.0.0 and 1 Subnet with 262142 Hosts.

Network address 172.16.0.0
Broadcast address 172.19.255.255
And Valid Host Range of 172.16.0.1 - 172.19.255.254

Hum, after a couple of beers and confirming with the TechExams.net IP Subnet Calculator, I'd say the answer is Yes.

dmaftei · August 2006

mikej412 wrote:

...and 1 Subnet with 262142 Hosts.

It's a supernet though...

I guess somebody should fix the calculator.

kwyap · August 2006

Anyone of you have done it before?

When i apply it on the bound interface, a message prompted
"Subnet mask must be greater or equal to the major net"

Theoritically it should work, right? Can anyone explain it?

EdTheLad · August 2006

kwyap wrote:

Anyone of you have done it before?

When i apply it on the bound interface, a message prompted
"Subnet mask must be greater or equal to the major net"

Theoritically it should work, right? Can anyone explain it?

Are you sure you changed the rip version to 2?It's just Ripv1 doesnt support cidr, i will check it later in my lab if this isnt the problem.

JVE · August 2006

I dont think you will ever get that to work. A class B, is the major network address and once you actually type a number like 172, you are forced to have at least 16 bits, no less. That would be like typing the number 10 and putting a 5 bit mask, the router will not accept that. Try a 10.x.x.x and put a mask that has 13 bits and it will work, but you wont get the router to accept a 14 bit mask as a summary route or any other thing with less then 16 bits of a class b address. At least I dont think so.

That calculator saying it is possible or not, I dont think it is right.

EdTheLad · August 2006

JVE, you think wrong, using a mask smaller than the major network is called
supernetting or cidr.

sprkymrk · August 2006

Ed is correct, it does work. My LAN has 2 class C subnets using a 255.255.254.0 mask behind a single router and it works just fine as a supernet. Been running this way with over 400 hosts just fine for a few years.

JVE · August 2006

ok, i just been going by examples and I have never seen them give such an example. I have no doubts you guys are right.

EdTheLad · August 2006

kwyap wrote:

Anyone of you have done it before?

When i apply it on the bound interface, a message prompted
"Subnet mask must be greater or equal to the major net"

Theoritically it should work, right? Can anyone explain it?

Ripv2 does support cidr but it is not supported with cisco's implementation.
So the message prompt you receive is the correct behaviour.

sprkymrk · August 2006

JVE wrote:

ok, i just been going by examples and I have never seen them give such an example. I have no doubts you guys are right.

Learning from each other is great, isn't it?
Just search through a few of my posts and you'll see how many times I have been corrected by folks who know more than me. I have been known to cause a superiority complex in others who hang around me

.

BubbaJ · September 2006

ed_the_lad wrote:

Ripv2 does support cidr but it is not supported with cisco's implementation.
So the message prompt you receive is the correct behaviour.

Cisco claims that it does:

Cisco's implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).

I tried this on a 12.3 version and it works fine when setting RIP to version 2 and disabling auto summarization.

EdTheLad · September 2006

BubbaJ wrote:

ed_the_lad wrote:

Ripv2 does support cidr but it is not supported with cisco's implementation.
So the message prompt you receive is the correct behaviour.

Cisco claims that it does:

Cisco's implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).

I tried this on a 12.3 version and it works fine when setting RIP to version 2 and disabling auto summarization.

I cant see where in the link you provided it says it supports specifying summarization less than the major net.
Check out summarization restrictions
Rip ios 12.4
I have 12.3(15) and it doesnt work for me.

!30 · September 2006

I think the answer is :

{ I only find you the sumarize for those 4 segment's } :

172.16.0.0/16
172.17.0.0/16
172.18.0.0/16
172.19.0.0/16

You can use this 172 ( I mean first 8 bite's are the same with all IP's segment ).Now let's see how many bite's from the second octect are the same with all four ( and then sum thos 8 + x byte's and get the sumariez ).

16 = 00010000
17 = 00010001
18 = 00010010
19 = 00010011

and get 8+6 mean 14 so the sumariez route is 172.16.0.0/14 .

I think this was helpful !

EdTheLad · September 2006

!30 wrote:

I think the answer is :

{ I only find you the sumarize for those 4 segment's } :

172.16.0.0/16
172.17.0.0/16
172.18.0.0/16
172.19.0.0/16

You can use this 172 ( I mean first 8 bite's are the same with all IP's segment ).Now let's see how many bite's from the second octect are the same with all four ( and then sum thos 8 + x byte's and get the sumariez ).

16 = 00010000
17 = 00010001
18 = 00010010
19 = 00010011

and get 8+6 mean 14 so the sumariez route is 172.16.0.0/14 .

I think this was helpful !

Maybe you should read the thread before responding, go and try summarize 172.16.0.0/14 on a real router using rip.Then when it doesnt work come back and read the thread again.

kwyap · September 2006

Study from BSCI, RIPv2 doesn't support discontiguos network. Is this the reason i couldn't summarise it to 14 bit of subnet mask?

EdTheLad · September 2006

kwyap wrote:

Study from BSCI, RIPv2 doesn't support discontiguos network. Is this the reason i couldn't summarise it to 14 bit of subnet mask?

This is not the reason why,ripv2 does support discontiguous networks when you disable auto-summary.Ripv1 however does not.The reason you cant is purely down to ios coding, for some reason the developer choose not to allow
supernet summarization.

BubbaJ · September 2006

ed_the_lad wrote:

BubbaJ wrote:

ed_the_lad wrote:

Ripv2 does support cidr but it is not supported with cisco's implementation.
So the message prompt you receive is the correct behaviour.

Cisco claims that it does:

Cisco's implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).

I tried this on a 12.3 version and it works fine when setting RIP to version 2 and disabling auto summarization.

I cant see where in the link you provided it says it supports specifying summarization less than the major net.
Check out summarization restrictions
Rip ios 12.4
I have 12.3(15) and it doesnt work for me.

It says it supports CIDR. I will do this again when I have access to my equipment. I'm not where I can try it again. I will try the exact addresses in the original question. I did this with some Class C addresses as a CIDR block.

mikej412 · September 2006

BubbaJ wrote:

It says it supports CIDR.

I tried it with those addresses and RIP V2 and got the same message.

From RFC 2453
supernet routes (routes with a netmask less specific than the "natural" network mask) must not be advertised where they could be misinterpreted by RIP-1 routers.

Looks like it might have been "programmer choice." Probably could have been programed to work in an all Version 2 network.... but changing documention is easier.

EdTheLad · September 2006

BubbaJ wrote:

It says it supports CIDR. I will do this again when I have access to my equipment. I'm not where I can try it again. I will try the exact addresses in the original question. I did this with some Class C addresses as a CIDR block.

Supernet advertisement (advertising any network prefix less than its classful major network) is not allowed in RIP route summarization, other than advertising a supernet learned in the routing tables. Supernets learned on any interface that is subject to configuration are still learned.

For example, the following summarization is invalid: (invalid supernet summarization)

Router(config)# interface Ethernet 1

Router(config-if)# ip summary-address rip 10.0.0.0 252.0.0.0

Which means CIDR is indirectly supported,but not supported in the way discussed in this thread.So cisco can justify their claim to support CIDR.

IP Summarization

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