Route Summarization

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Sep 9th, 2007
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Hello all, I am studying for my CCNP and am going through different scenarios on my testebd at home. I am trying to perform route summarization on a few subnets I have configured to some loopback interfaces. I have the following IPS:


Loopback0 - 10.0.10.0 255.255.255.0

Loopback1 - 10.0.11.0 255.255.255.0

Loopback2 - 10.0.12.0 255.255.255.0

Loopback3 - 10.0.13.0 255.255.255.0


I want to summarize those 4 routes out the Serial 0 interface. I am running EIGRP with no autosummarization in place. I have the following network statements in there:


network 10.0.10.0 0.0.0.255

network 10.0.11.0 0.0.0.255

network 10.0.12.0 0.0.0.255

network 10.0.13.0 0.0.0.255


Any help you can provide would be very helpful. I have researched it for a while and found nothing. I now turn to the expertise of my colleagues.


Thanks,


Tim

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akimalijbsk8r Sun, 09/09/2007 - 12:02
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Excuse me if I'm wrong. But let me preface my answer with the fact that I'm getting ready to take my CCNA at the end of the month. That being said, wouldn't those routes get summarized as follows...


network 10.0.8.0 0.0.7.255


Akim

Richard Burts Sun, 09/09/2007 - 13:10
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Akim


You are correct that the optimum address block that includes these addresses is 10.0.8.0/29.


And your statement would be effective if the intent was to include all interfaces with a single command. But it does not summarize anything.


Tim

To summarize on the serial interface you need this statement under the serial interface:

ip summary-address eigrp 10.0.8.0 255.255.248.0


HTH


Rick

timothykerr Sun, 09/09/2007 - 18:20
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Hi Rick,

Thank you for your response as well. This would also work (and I have done it before), however, it would also include more routes than I want to summarize. I realize that the subnets I picked are not ideal for proper route summarization. However, that is why I selected them. I want to know if there is a way to do that.

If you have any other thoughts, they would be greatly appreciated. Is there a way to do it with using a route-map?


Thanks,


Tim

timothykerr Sun, 09/09/2007 - 18:16
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Akim,

Yes they would. However, that would also summarize 10.0.8.0 - 10.0.13.0 as well. I want to know if there is a way to only summarize the 4 routes that I selected. I tried using 10.0.10.0 0.0.3.255, but that did not work. That summarized 10.0.8.0-10.0.11.0. Thanks for your reply though.

Richard Burts Sun, 09/09/2007 - 18:28
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Tim


Because of the particular subnets that you chose there is not any way to summarize just those 4 subnets. And there is not anything in a route map that can make it better.


One of the basic rules of IP addressing and summarization is that you can only summarize blocks of addresses that are a power of 2 and that start on a bit boundary. Your example of 4 subnets is a power of 2 but it does not start on a bit boundary. For a block of 4 subnets the bit boundaries are 0, 4, 8, 12, etc. Since you started at 10 the best that you can do is to summarize a block of 8 addresses. And that includes more subnets than you want. But it is the best that you can do given your choice of subnets.


HTH


Rick

Edison Ortiz Sun, 09/09/2007 - 18:30
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You can accomplish this with 2 summary routes.


ip summary-address eigrp 1 10.0.12.0 255.255.254.0 5

ip summary-address eigrp 1 10.0.10.0 255.255.254.0 5


but you can't with a single summary route, if you are concerned with overlapping.

Richard Burts Sun, 09/09/2007 - 18:38
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Edison


You make an excellent point. 2 summary addresses with each summarizing 2 subnets will summarize exactly the desired subnets. My response focused on summarizing with a single statement which I thought was the point of the original post and of the first response. But you point out that there is an alternative that will work. Excellent point.


HTH


Rick

Edison Ortiz Sun, 09/09/2007 - 18:43
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Yup, agreed.


The whole purpose for route summarization is to reduce the size of the routing table. Ideally, you want to summarize within the network boundaries and avoid overlapping. If it takes more than one command, so be it :)

timothykerr Sun, 09/09/2007 - 18:46
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Thank you both for your responses. I was hpoing to accomplish this with one subnet, but Edison, you bring up a good point with the two subnets. Thank you, also, Rick for your explanations. That is why I post at these forums, because you have such professionals that are willing to take the time to answer your questions. Thanks again.

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