BGP As override

Answered Question
Oct 17th, 2009
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hello friends,

pls explain as-override feature in brief if possible please give me the example

Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Sourabh,


neigh x.x.x.x as-override allows to a PE router to change the private ASN used by a CE node on an eBGP session running on a VRF access link.

the private ASN is changed in the PE ASN.


another CE node on another VRF (=connected to another PE node) will see the eBGP route coming from first site with an AS path of:


provider-ASN provider-ASN


instead of:

provider-ASN site1-ASN


this allows to use the same private ASN on all sites of a customer.

That is a clear management advantage for a service provider.


This because BGP by default doesn't accept BGP routes with an AS path attribute that contains local AS number.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Sat, 10/17/2009 - 10:20
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Hello Sourabh,


neigh x.x.x.x as-override allows to a PE router to change the private ASN used by a CE node on an eBGP session running on a VRF access link.

the private ASN is changed in the PE ASN.


another CE node on another VRF (=connected to another PE node) will see the eBGP route coming from first site with an AS path of:


provider-ASN provider-ASN


instead of:

provider-ASN site1-ASN


this allows to use the same private ASN on all sites of a customer.

That is a clear management advantage for a service provider.


This because BGP by default doesn't accept BGP routes with an AS path attribute that contains local AS number.


Hope to help

Giuseppe


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