Enforces the use of the first autonomous system (AS) path for external BGP (eBGP) routes.
The device does not require the first AS listed in the AS_SEQUENCE field of an AS path update message from eBGP neighbors be the AS of the neighbor that sent the update.
BGP configuration mode
The no form of the command disables this feature.
This command causes the router to discard updates received from eBGP peers that do not list their AS number as the first AS path segment in the AS_PATH attribute of the incoming route.
The device accepts the update only if the AS numbers match. If the AS numbers do not match, the device sends a notification message to the neighbor and closes the session. This requirement applies to all updates received from eBGP neighbors.
The following example configures the device to enforce the use of the first AS path.
device# configure terminal device(config)# router bgp device(config-bgp-router)# enforce-first-as