Please opine on this summarization finding

Answered Question
Mar 3rd, 2010
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Hi folks,


Someone claimed that is possible to summarize the networks listed below in (4) aggregate-address statements (to be advertised via BGP and also to be placed in prefix-lists). I tried to do that and I do not see how. In my view, Option 2) is the correct, most compact and most accurate way to represent this.


I'd appreciate if you can confirm my finding is correct or if you can come up with a better summarization than this please let me know because I am very curious. Thanks!!!


1.77.206.0 255.255.255.0
1.77.207.0 255.255.255.0
1.77.210.0 255.255.255.0
1.77.211.0 255.255.255.0
1.77.212.0 255.255.255.0
1.77.213.0 255.255.255.0
1.77.214.0 255.255.255.0
1.77.215.0 255.255.255.128
1.77.216.0 255.255.252.0
1.77.217.0 255.255.255.0




Option 1)

!** I would include more addresses than it should be advertised (on .215.0 and .216.0)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.252.0
aggregate-address 1.77.216.0 mask 255.255.254.0


Option 2)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.254.0
aggregate-address 1.77.214.0 mask 255.255.255.0
aggregate-address 1.77.215.0 mask 255.255.255.128
aggregate-address 1.77.216.0 mask 255.255.252.0
aggregate-address 1.77.217.0 mask 255.255.255.0

Correct Answer by Giuseppe Larosa about 7 years 4 months ago

Hello Marlon,

generally speaking

when creating summary routes we can mean two different types of summary routes:

exclusive summary routes that contain only the listed component routes

non exclusive summary routes that can contain other component  routes.


the second type of summary routes can be acceptable in some scenarios: private IP addressing the missing routes are not existing in your enterprise so no issue arises from their inclusion in the summary routes.


So I see here this possible different interpretation of summary routes.


your option 2) is a set of exclusive summary routes, option 1) is a set of non exclusive summary routes clearly shorter.


Hope to  help

Giuseppe

Correct Answer by Jon Marshall about 7 years 4 months ago

news2010a wrote:


Option 1)

!** I would include more addresses than it should be advertised (on .215.0 and .216.0)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.252.0
aggregate-address 1.77.216.0 mask 255.255.254.0


Option 2)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.254.0
aggregate-address 1.77.214.0 mask 255.255.255.0
aggregate-address 1.77.215.0 mask 255.255.255.128
aggregate-address 1.77.216.0 mask 255.255.252.0
aggregate-address 1.77.217.0 mask 255.255.255.0


Marlon


Hope that "someone" was not me


You are correct in what you say. The first option includes more addresses than you want advertised.


Not sure if there is a typo but in your original list of networks to be summarised -


1.77.216.0 255.255.252.0 
1.77.217.0 255.255.255.0


1.77.216.0 255.255.252.0  = 1.77.216 -> 1.77.219.  so you wouldn't need the 1.77.217.0 entry. Was the 216 entry meant to have a /24 subnet mask ?


Jon

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Correct Answer
Jon Marshall Wed, 03/03/2010 - 23:51
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news2010a wrote:


Option 1)

!** I would include more addresses than it should be advertised (on .215.0 and .216.0)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.252.0
aggregate-address 1.77.216.0 mask 255.255.254.0


Option 2)
aggregate-address 1.77.206.0 mask 255.255.254.0
aggregate-address 1.77.210.0 mask 255.255.254.0
aggregate-address 1.77.212.0 mask 255.255.254.0
aggregate-address 1.77.214.0 mask 255.255.255.0
aggregate-address 1.77.215.0 mask 255.255.255.128
aggregate-address 1.77.216.0 mask 255.255.252.0
aggregate-address 1.77.217.0 mask 255.255.255.0


Marlon


Hope that "someone" was not me


You are correct in what you say. The first option includes more addresses than you want advertised.


Not sure if there is a typo but in your original list of networks to be summarised -


1.77.216.0 255.255.252.0 
1.77.217.0 255.255.255.0


1.77.216.0 255.255.252.0  = 1.77.216 -> 1.77.219.  so you wouldn't need the 1.77.217.0 entry. Was the 216 entry meant to have a /24 subnet mask ?


Jon

Correct Answer
Giuseppe Larosa Thu, 03/04/2010 - 05:06
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Hello Marlon,

generally speaking

when creating summary routes we can mean two different types of summary routes:

exclusive summary routes that contain only the listed component routes

non exclusive summary routes that can contain other component  routes.


the second type of summary routes can be acceptable in some scenarios: private IP addressing the missing routes are not existing in your enterprise so no issue arises from their inclusion in the summary routes.


So I see here this possible different interpretation of summary routes.


your option 2) is a set of exclusive summary routes, option 1) is a set of non exclusive summary routes clearly shorter.


Hope to  help

Giuseppe

news2010a Thu, 03/04/2010 - 08:18
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What an answer. Thanks Giuslar and Jon for the insight.


Jon, it was not a typo. All network masks are correct as presented.

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