BGP AS-Path elimination

Answered Question
Sep 27th, 2010
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Hi all experts.


I have 3 AS connected to each other


AS-1--------------------------------AS-2-------------------------------------AS-3


I have a prefix that is being originated from AS-1 i.e. 10.1.1.0/24. When the EBGP router in AS-3 will get this prefix, its AS-Path will be


1 2


Now, is there any possible configuration that can be done in AS-2, such that, EBGP router is AS-3 gets this


2


Meaning, is there any possible configuration in BGP such that an AS-Path can be completely eliminated ?  for example AS-Path in this case ?

Correct Answer by Jon Marshall about 6 years 10 months ago

Jonn.cos88 wrote:


Hi all experts.


I have 3 AS connected to each other


AS-1--------------------------------AS-2-------------------------------------AS-3


I have a prefix that is being originated from AS-1 i.e. 10.1.1.0/24. When the EBGP router in AS-3 will get this prefix, its AS-Path will be


1 2


Now, is there any possible configuration that can be done in AS-2, such that, EBGP router is AS-3 gets this


2


Meaning, is there any possible configuration in BGP such that an AS-Path can be completely eliminated ?  for example AS-Path in this case ?


Jon


There really is no specific way to do this in BGP as there is to add an AS-PATH with AS-PATH prepending. Nor should there be really as it is the AS-PATH that ensures loop free routing with EBGP.


There is a command to remove private AS numbers in the AS-PATH eg.


router bgp

neighbor x.x.x.x remove-private-as


will remove all private AS numbers but your numbers ie. 1/2/3 are public AS numbers.


The workaround would be to advertise a summary address from the AS-2 router which would remove the AS numbers behind that router but that is not particularly useful for a 10.1.1.0/24 network.


Jon

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Correct Answer
Jon Marshall Mon, 09/27/2010 - 12:41
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Jonn.cos88 wrote:


Hi all experts.


I have 3 AS connected to each other


AS-1--------------------------------AS-2-------------------------------------AS-3


I have a prefix that is being originated from AS-1 i.e. 10.1.1.0/24. When the EBGP router in AS-3 will get this prefix, its AS-Path will be


1 2


Now, is there any possible configuration that can be done in AS-2, such that, EBGP router is AS-3 gets this


2


Meaning, is there any possible configuration in BGP such that an AS-Path can be completely eliminated ?  for example AS-Path in this case ?


Jon


There really is no specific way to do this in BGP as there is to add an AS-PATH with AS-PATH prepending. Nor should there be really as it is the AS-PATH that ensures loop free routing with EBGP.


There is a command to remove private AS numbers in the AS-PATH eg.


router bgp

neighbor x.x.x.x remove-private-as


will remove all private AS numbers but your numbers ie. 1/2/3 are public AS numbers.


The workaround would be to advertise a summary address from the AS-2 router which would remove the AS numbers behind that router but that is not particularly useful for a 10.1.1.0/24 network.


Jon

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