300-410 Layer 3 Technologies Practice Question
A network architect is designing a BGP routing policy to influence inbound traffic from an ISP. The architect wants to make a specific prefix less preferred by the ISP by prepending the AS path multiple times. Which BGP attribute should be manipulated to achieve this?
⚠ Common exam trap
A common mix-up: candidates confuse attributes used for inbound versus outbound traffic influence, or assuming MED is always effective for inbound traffic.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
AS_PATH
AS_PATH prepending is a technique where the local AS number is added multiple times to the AS_PATH attribute of a route advertised to external peers. This increases the path length, making the route less attractive to the receiving ISP, as BGP prefers shorter AS paths. This is a standard method for influencing inbound traffic in a multi-homed environment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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LOCAL_PREF
Why it's wrong here
LOCAL_PREF is a well-known discretionary attribute used within an autonomous system to influence outbound traffic. It is not propagated to external BGP peers, so it cannot be used to influence inbound traffic from an ISP. It is typically used to prefer one exit point over another within the local AS.
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MED
Why it's wrong here
MED (Multi-Exit Discriminator) is an optional non-transitive attribute that can influence inbound traffic from an adjacent AS by suggesting a preferred entry point. However, it is only a hint and is often ignored by ISPs. AS path prepending is a more forceful and widely supported method to influence inbound traffic.
- ✓
AS_PATH
Why this is correct
AS_PATH is a well-known mandatory attribute that lists the autonomous systems a route has traversed. By prepending additional AS numbers to the AS_PATH, the path length increases, making the route less preferred by BGP's best path selection process. This is a common method to influence inbound traffic, as the ISP will prefer shorter AS paths.
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NEXT_HOP
Why it's wrong here
NEXT_HOP is a well-known mandatory attribute that indicates the next-hop IP address for a route. It is not used to influence path selection based on preference; rather, it is used for reachability. Manipulating NEXT_HOP would not make a prefix less preferred; it could cause routing loops or black holes if set incorrectly.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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