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AS Override

in BGP, blog on January 27, 2015

AS Override

The AS override feature allows a provider edge (PE) router to change the private autonomous system (AS) number used by a customer edge (CE) device on an external BGP (EBGP) session running on a VPN routing and forwarding (VRF) access link. The private AS number is changed to the PE AS number. Another CE device connected to another PE device sees the EBGP route coming from the first site with an AS path of provider-ASN provider-ASN, instead of provider-ASN site1-ASN. This allows enterprise networks to use the same private ASN on all sites.

The AS override feature offers a clear management advantage to the service provider because BGP by default does not accept BGP routes with an AS path attribute that contains the local AS number.

In an enterprise network with multiple sites, you might wish to use a single AS number across sites. Suppose, for example that two CE devices are in AS 64512 and that the provider network is in AS 65534.

When the service provider configures a Layer 3 VPN with this setup, even if the MPLS network has routes towards Device CE1 and Device CE2, Device CE1 and Device CE2 do not have routes to each other because the AS path attribute would appear as 64512 65534 64512. BGP uses the AS path attribute as its loop avoidance mechanism. If a site sees its own AS number more than once in the AS path, the route is considered invalid.

One way to overcome this difficulty is with the as-override statement, which is applied to the PE devices. The as-override statement replaces the CE device’s AS number with that of the PE device, thus preventing the customer AS number from appearing more than once in the AS path attribute.

If a customer uses AS path prepending to make certain paths less desirable and the service provider uses AS override, each CE AS number occurrence in the AS-path is changed to the service provider AS number. For example, suppose that all customer sites use the same AS number, say 64512. If the ISP uses AS number 65534, one customer site sees the path to another site as 65534 65534. If the customer prepends 64512 on a particular path to make it less desirable, another customer site sees that path as 65534 65534 65534.


Categories: BGP blog


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