350-501 Architecture Practice Question
A service provider is troubleshooting BGP route selection for prefixes received from two different peers. The first peer prepends its AS path twice, making it longer than the second peer's path. However, the router still prefers the route with the longer AS path. Which additional attribute could cause this behavior?
⚠ Common exam trap
Cisco often tests the order of BGP path selection attributes, and the trap here is that candidates mistakenly think AS path length is always the deciding factor, forgetting that local preference (or weight) is evaluated first and can override a shorter AS path.
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
✓
The route has a higher local preference
BGP selects the best path based on a sequence of comparison steps. The first step after considering the next-hop reachability is to prefer the route with the highest weight (Cisco proprietary), then the highest local preference. Since the router prefers the longer AS path, a higher local preference (option D) on the first peer's route overrides the shorter AS path of the second peer, as local preference is evaluated before AS path length in the BGP best-path selection algorithm.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The route has a lower MED
Why it's wrong here
MED influences only when paths are from the same AS; it does not override AS path length.
- ✗
The route has a higher weight
Why it's wrong here
Weight is considered before local preference, but if both routes have default weight (0), local preference decides. However, if one has higher weight, it would be preferred, but that would also override local preference; but typically weight is not set in this scenario. Nonetheless, local preference is the more common cause.
- ✗
The route has a lower origin type
Why it's wrong here
Origin is considered after AS path length; it cannot override a longer AS path.
- ✓
The route has a higher local preference
Why this is correct
Local preference is the first criterion in BGP path selection; a higher value will be chosen regardless of AS path length.
- ✗
The route has a lower neighbor router ID
Why it's wrong here
Router ID is a tiebreaker after all other attributes; it does not override AS path length.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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