The check-first-as command enables the function to check the first AS number in the AS_Path list that is carried in the Update message sent by the EBGP peer.
The undo check-first-as command disables the function.
By default, the function is enabled.
Usage Scenario
By default, BGP checks the first AS number in the AS_Path list that is carried in the Update message sent by the EBGP peer. If only the first AS number indicates the AS where the EBGP peer locates, the Update message is accepted. Otherwise, the Update message is denied, and the EBGP connection goes Down.
Precautions
The check-first-as command is not listed in the configuration file.
After the undo check-first-as command is configured, loops have a greater chance to occur. Therefore, use the command with caution.
Follow-up Procedure
After the configuration is complete, run the refresh bgp command if you want to check the received routes again.