Verifying an AS Path

You can use the bgp enforce-first-as command to cause BGP to compare the first AS in the AS-path of a received route with the configured remote AS number of that EBGP peer. If the check fails, BGP returns a notification message to the peer.

bgp enforce-first-as

• Use to cause BGP to determine whether the first AS in the AS path of a route received from an EBGP peer matches the remote AS number of that peer.
• If the AS does not match, BGP sends a notification to the peer with the error code “ update message error” and error subcode “ malformed as-path.”
• This feature is disabled by default.
• Example

  host1(config-router)#bgp enforce-first-as

• Causes BGP to check the AS path of all routes received after you issue the command.

  To apply the new behavior to routes that are already present in the BGP routing table, you must use the clear ip bgp command to perform a soft clear or hard clear of the current BGP session.

• Use the no version to prevent the AS comparison from taking place.
• See bgp enforce-first-as.

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