300-410 Layer 3 Technologies Practice Question
A network engineer is configuring a Cisco IOS XE router to connect to an ISP via BGP. The engineer wants to influence inbound traffic from the ISP by prepending the router's AS number multiple times to the BGP updates sent to the ISP. Which BGP attribute should the engineer modify to achieve this?
⚠ Common exam trap
Many candidates confuse attributes that influence outbound traffic (local preference, weight) with those that influence inbound traffic (AS path, MED).
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 the technique of adding multiple instances of the local AS number to the AS path attribute in BGP updates sent to external peers. This makes the route less attractive because a longer AS path is less preferred. It is a standard method to influence inbound traffic from an ISP. Local preference and weight affect outbound traffic, while MED is an alternative but less forceful method for inbound influence.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Weight
Why it's wrong here
Weight is a Cisco-specific attribute that is local to the router and not advertised to any BGP peer. It is used to influence outbound path selection on the local router only. Since it is not sent to the ISP, it cannot affect inbound traffic. Modifying weight would only change the router's own decision for outgoing packets, not how the ISP routes traffic back.
- ✓
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 engineer makes the route appear longer, which is less preferred by BGP. This influences inbound traffic because the ISP will choose a shorter AS path if available. It is a common method to manipulate inbound traffic.
- ✗
Multi-exit discriminator (MED)
Why it's wrong here
MED is an optional non-transitive attribute used to suggest a preferred entry point into an AS that has multiple connections. It is sent to external peers but is typically only considered between directly connected autonomous systems. However, MED is not as widely respected as AS-path prepending, and it influences inbound traffic only if the ISP honors it. It does not prepend AS numbers.
- ✗
Local preference
Why it's wrong here
Local preference is a well-known discretionary attribute used within an autonomous system to influence outbound traffic from that AS. It is not advertised to external BGP peers, so it cannot affect inbound traffic from the ISP. Modifying local preference would affect which exit point internal routers choose, but not how the ISP sends traffic toward the network.
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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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