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Routing & Switching · Advanced

AS-Override

A PE-side BGP option that replaces the customer's ASN in the AS_PATH with the provider's ASN, letting sites with the same ASN learn each other's routes.

Definition

Used in MPLS L3VPN when all customer sites use the same private ASN. The PE substitutes its own ASN, so the receiving CE does not detect a loop. Site of Origin can be used to prevent loops at multihomed sites.

Example

neighbor 192.0.2.2 as-override under address-family ipv4 vrf CUST

Exam tip

as-override on the PE vs allowas-in on the CE solve the same problem.

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