True or False: The BGP AS-path is not guaranteed to always be loop free.

• A. Depends on configuration
• B. Not enough information
• C. True
• D. False

Answer: (D)

BGP uses the AS_PATH as a built-in loop prevention mechanism. As a route traverses the network, each AS adds its number to the AS_PATH. When a router receives a route update, it checks whether its own AS is already present in that path; if it is, the update is rejected to prevent a loop. This design makes AS_PATH inherently loop-free in normal operation. So saying the AS_PATH is not guaranteed to be loop free isn’t correct—the path is kept loop-free by this check. In special cases like confederations or route reflectors, you may see tokens in the path, but the fundamental rule remains: a router will not accept or re-advertise a route that would create a loop involving its own AS.