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Route Summarization

Posted on 2016-08-04
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Last Modified: 2016-08-07
If I understand Route Summarization if to reduce the size of the Routing table. I wonder why we need to summarize instead of just using Default route ?



Thank you
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Question by:jskfan
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by:Richardson Porto
Richardson Porto earned 100 total points
ID: 41743611
Because your summarized routes may not point to the same IP address of your default route.
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by:jskfan
ID: 41744286
Let 's say we have
192.168.4.0/26  I believe this will cover subnets 192.168.4.192 to 192.168.4.254
192.168.5.0/27
192.168.6.0/28
192.168.7.0/29
192.168.9.0/28

If I am not wrong this will be summarized as 192.168.4.0 0.0.0.64
 
If so can we summarize it as 192.168.4.0 0.0.0.255 . This will have some extra routes in the table but I am not sure if it breaks anything.
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by:jskfan
ID: 41744302
Per this tool
https://nubem.com/ip-summarization-tool/

it is : 192.168.4.0      /22
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by:Predrag Jovic
ID: 41744672
As Richardson pointed out location of summarized route does not have to match path for default route.
If I am not wrong this will be summarized as 192.168.4.0 0.0.0.64
That is not summarization at all (at least not valid one). :)
You need to advertise one subnet for all networks
192.168.4.0/26

192.168.5.0/27
192.168.6.0/28
192.168.7.0/29
192.168.9.0/28
192.168.4.0/22 will not cover all since you have 192.168.9.0/28
192.168.4.0/22  <- IP address range 192.168.4.0 - 192.168.7.255
In this case you can summarize 192.168.4.0/22 (192.168.0.0 0.0.3.255) and leave 192.168.9.0/28 not summerized.
Summarization for whole range would be 192.168.0.0/20 (192.168.0.0 0.0.15.255).
That's why it is really important to choose carefully IP address ranges from the very beginning, to avoid to broad summarization.
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by:jskfan
ID: 41745826
I guess I missed to enter this one in the Summarization Tool
192.168.9.0/28

Well, the one thing I wanted to understand , if summarization is broader than what it is supposed to be,  what impact will it have ?
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Predrag Jovic earned 400 total points
ID: 41745917
Then you will advertise more network ranges that you don't have in your network. It can overlap with some existing network that you have somewhere else in your network (bad routes - so traffic can end up on this router and not where it suppose to go) and (the case that routes are not present somewhere else) that can lead to unnecessary traffic sent from other routers that will be dropped when reaches router that is advertising summarized route (since you are advertising network ranges that you don't have).
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