Why do BGP routers reject an update that contains their own AS in the path?

• A. It would cause a loop
• B. It would be ignored due to TTL
• C. It would cause longer path
• D. It would violate policy

Answer: (A)

BGP uses the AS_PATH attribute to prevent routing loops. Each time a route is advertised, the AS adds its own identifier to the path. If a router receives an update that already contains its own AS in the AS_PATH, accepting or re-advertising that route would loop packets back through the same AS sequence. To avoid this, the router discards the update. TTL differences or path length alone don’t enforce this loop prevention, and while policy can influence filtering, the standard reason here is to stop potential loops detected by the AS_PATH check.