Difference between AS Override and Allowas In

1. AS Override :
Its feature allows a provider edge (PE) router to change private autonomous system used by customer edge (CE) device on an external BGP session running on a VPN routing and forwarding access link. The private AS number is changed to PEAS number.

2. Allowas In :
This feature allows for routes to be received and processed even if router detects its own ASN in AS-Path. A router discards BGP network prefixes if it sees its ASN in AS-Path as a loop prevention mechanism.

Difference between AS Override and Allowas In :

Parameter AS Override Allowas In
Overview It is used by PE to modify AS path. This is done so that prefix is not dropped on default behavior of BGP to disallow its own AS member in AS path of prefix. It is used as an AS path loop prevention mechanism where default behavior of BGP is to not allow its own AS number in AS Path of prefix.
Output AS Override feature replaces AS in BGP updates to local AS. It updates in the BGP that contains local AS is permitted by Allowace In.
Configured at Its feature is configured at Service Provider End. Its feature is configured on Customer End.
Configuration It is configured under:
router bgp
neighbor as-override
Configured under
router bgp
neighbor allowas-in
AS Path There is modification in AS path. There is no modification in AS path.
Scope. Configuration performed at Service provider end. Configuration performed at Service provider end.
Most optimal use. This approach is best used when customer requires to keep minimum configuration at CE side. This approach is best used to introduce an exception in BGP as a loop prevention mechanism.

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