Difference between AS Override and Allowas In

The Border Gateway Protocol (BGP) features AS Override and Allowas In are used to control how BGP routing information is exchanged between Autonomous Systems (AS). These mechanisms help network administrators handle specific scenarios where standard BGP loop prevention rules need to be bypassed for legitimate network designs.

AS Override

AS Override is a BGP configuration option that allows a border router in one AS to replace the AS number of the directly connected AS when advertising routes to another AS. This is particularly useful in service provider networks where customers use private AS numbers or in hub-and-spoke topologies.

For example, if AS1 is directly connected to AS2, but AS2 uses a private AS number, AS1 can use AS Override to advertise routes to AS3 using AS1's AS number instead of AS2's private number. This ensures proper connectivity while maintaining AS path integrity.

Features of AS Override

• Allows multihomed networks to advertise the same prefix to multiple upstream providers

• Modifies the AS path attribute to reflect the advertising AS number instead of the originating AS

• Helps service providers manage customer AS numbers in MPLS VPN environments

Allowas In

Allowas In is a BGP configuration option that allows a border router to accept routes that contain its own AS number in the AS path. By default, BGP rejects such updates to prevent routing loops, but enabling Allowas In permits the router to accept these updates under controlled conditions.

This is valuable in scenarios where a BGP router receives updates from a neighboring AS that traverses back through its own AS. For example, in hub-and-spoke topologies where spoke sites need to communicate through the hub.

Features of Allow-AS-in

• Allows an AS to accept BGP routes containing their own AS number in the AS path

• Useful in hub-and-spoke configurations and confederation scenarios

• Can specify maximum occurrences of own AS number to prevent excessive loops

Comparison

Conclusion

AS Override and Allowas In are essential BGP features that provide flexibility in network design by selectively bypassing standard loop prevention mechanisms. AS Override modifies outbound advertisements for service provider scenarios, while Allowas In accepts inbound routes containing the local AS number for hub-and-spoke topologies.