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RIPv2 Auto Summary

Answered Question
Jul 8th, 2013
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I'm trying to understand how auto summarization works in RIPv2. I understand that it is enabled by default. Does this mean that if I have a subnet mask (lets say 26) that is greater than the class subnet mask (24), that it will automatically summarize it down to the class mask (24)? And if so, isn't this an issue on discontiguous networks where a router in the middle would summarize all the subnets up to a classful netmask?

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Correct Answer by InayathUlla Sharieff about 4 years 1 month ago

Zane,

Yes correct.


HTH

Regards,

Inayath

*Plz rate all usefull posts and mark the thread as answered if it solved your query.

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InayathUlla Sharieff Mon, 07/08/2013 - 17:44
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Zane,

Yes your understanding is perfectly correct. Hence we recommend to turn off the auto summarization on RIPv2 and EIGRP protocols to avoid running in to discontigous subnet issue.


some good explanation;

http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfrip.html



https://learningnetwork.cisco.com/thread/11301



HTH

Regards

Inayath

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Zane Zakraisek Mon, 07/08/2013 - 17:58
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Why does RIPv2 have this enabled by default? I thought that the whole purpose of v2 was to be able to have discontiguous networks and VLSMs.

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InayathUlla Sharieff Mon, 07/08/2013 - 18:19
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RIPv2 is a classless routing protocol regardless of the use of the "no auto-summary" command.  The "no auto-summary" command is used to disable auto summarization at major network boundaries and is need to support discontiguous networks.  The "auto-summary" command would be used if you wanted to perform automatic summarization at major network boundaries


Auto Summarization is turned on by default for RIPv2 and EIGRP, altough these are Classless Routing protocols. So you manually have to make them Classless with the "no auto-summary" command.


But if you ask me the reason why they have done like that I dont have any comment to say why they have build in that way.


HTH

Regards

Inayath

Zane Zakraisek Mon, 07/08/2013 - 18:27
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So you say that the auto summary command would be used if you wanted to perform automatic summarization at major network boundaries, but if your summarizing at the major network, isn't that making it a classful protocol?

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InayathUlla Sharieff Mon, 07/08/2013 - 18:37
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zane,

RIPv2 and EIGRP are "classless" regardless of the use of the "no auto-summary" command.


Classful protocols do NOT send subnet mask information.


Classless protocols DO send subnet mask information.


This has nothing to do with the auto-summary command.


Auto-summary - summarizes at the major network boundaries.


No auto-summary - does NOT summarize at the major network boundaries.


Classful bit boundaries are defined by the Classes A, B, and C.  Example:


Class A Network 10.0.0.0/8

Class B Network 172.16.0.0/16

Class C Network 192.168.11.0/24


Major network boundaries represent the different "major" networks within each classful space.  Example:


Class A Network 10.0.0.0/8

Class A Network 61.0.0.0/8


two different "major" networks within the Class A address space.


172.16.0.0/16

172.24.0.0/16


two different "major" networks within the Class B address space.


192.168.11.0/24

192.168.63.0/24


two different "major" networks within the Class C address space.


When you configure either RIPv2 or EIGRP (BGP too) with the "auto-summary" command, you will automatically summarize when you cross these different "major" network boundaries'.


Lab it up and give it a try.


Hope this helps.


Regards

Inayath

Zane Zakraisek Mon, 07/08/2013 - 18:47
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Ok I think I've got it. I had a misunderstanding with some of the basics. So with RIPv2 and auto summarization enabled, you can have classless networks with any netmask, but when your routing between major networks it will summarize to a lower netmask to basically supernet the network and reduce the amount of entries in the routing tables? Is that about right?

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Correct Answer
InayathUlla Sharieff Mon, 07/08/2013 - 20:32
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Zane,

Yes correct.


HTH

Regards,

Inayath

*Plz rate all usefull posts and mark the thread as answered if it solved your query.

Zane Zakraisek Mon, 07/08/2013 - 21:00
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Thanks for baby stepping it with me. I appreciate it :)

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InayathUlla Sharieff Mon, 07/08/2013 - 22:52
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Zane,

Yes you are indeed correct. Look at the example below i have just snipp the outputs;


R1-------------------R2( F0/0 ip address 172.168.1.1 255.255.254.0)


R

1- With no auto summary the outputs will be like this:

router rip

version 2

no auto-summary


R1r#sh ip rip d

172.168.0.0/19    auto-summary

172.168.0.0/19

    [1] via 192.168.1.1, 00:00:17, FastEthernet0/0


Router#show ip route

R       172.168.0.0/19 [120/1] via 192.168.1.1, 00:00:05, FastEthernet0/0




2- With auto summary

router rip

version 2




R1r#sh ip rip da

172.168.0.0/16    auto-summary >>>>>>>>>>>Auto summary.

172.168.0.0/16

    [1] via 192.168.1.1, 00:00:19, FastEthernet0/0


Router#show ip route

R    172.168.0.0/16 [120/1] via 192.168.1.1, 00:00:11, FastEthernet0/0


Hope this helps


Regards

Inayath

Zane Zakraisek Mon, 07/08/2013 - 23:40
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So from what I've got out of all this is that auto summarization is an old technique used primarily in classful networks, and should be disabled, especially if
1. The networks are discontinuous
2. You have a netmask lower than the old one associated with the IP address class. (Ex a 192 network with a netmask lower than 24, or a 172 network with a netmask lower than 16)

Is this all correct. I'm sorry but I'm 18 and studying for my CCNA haha

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InayathUlla Sharieff Mon, 07/08/2013 - 23:48
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Zane,

Yes auto summarization is the old technique.


It should be disabled if you are using the subnetting/VLSM. Hope this clears.


HTH

Regards

Inayath

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Zane Zakraisek Mon, 07/08/2013 - 22:37
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I apologize but I just have one more clarifying question.

Does the CLASS of the network play a part in the auto summary. I understand that if I have auto summarization enabled it would summarize /28 networks up to /24, or /18 up to /16, but in all your examples you used up networks that fell into the old ip class ranges. What I'm asking is if I had routes like 192.168.0.0/18 and 192.168.0.0/20, would these summarize to 192.168.0.0/16.

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