Solved: BGP AS-Path elimination

Jonn cos · ‎09-27-2010

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 1 in this case ?

Jon Marshall · ‎09-27-2010

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 1 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

View solution in original post

Jon Marshall · ‎09-27-2010

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 1 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