What is AS_PATH prepending and how can it be used to influence routing for security or policy purposes?

• A. Adds extra AS numbers to AS_PATH to lengthen; deter usage or steer traffic or provide a preferred path for customer security.
• B. Removes AS numbers to shorten path.
• C. Encrypts AS_PATH to hide topology.
• D. Sets a global MED value.

Answer: (A)

AS_PATH prepending is the act of adding extra occurrences of your own AS to the AS_PATH attribute in BGP announcements. This makes the advertised route appear longer to other networks, and since many routers prefer shorter AS_PATHs, the path with the additional repeats becomes less attractive for inbound traffic.

This technique is useful for security or policy reasons because you can steer inbound traffic away from certain paths or ASes and toward paths you trust or control more tightly. For example, you might prepend to push traffic to a preferred upstream or avoid passing through a source you don’t want to route through, aligning traffic flows with security or business policies. It’s a simple, manual form of traffic engineering based on path length, and it’s not guaranteed: some networks ignore prepends, apply additional policies, or override decisions with other routing factors.

It’s not about removing AS numbers, encrypting the AS_PATH, or setting a MED value, which are different mechanisms or attributes with separate purposes.