350-501 BGP community Practice Question
A service provider wants to influence inbound traffic from a customer AS. The customer AS is multi-homed to two provider ASes. Which BGP community is most effective to make the customer prefer a specific entry point?
⚠ Common exam trap
Candidates often confuse the purpose of BGP communities. 'no-export' prevents advertisement to external peers, while 'no-advertise' prevents any advertisement. Additionally, local preference is an attribute set on the receiving end, not a community.
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
✓
Set community 'no-export' on routes from the undesired entry point.
Setting the BGP 'no-export' community on routes from the undesired entry point prevents those routes from being advertised to the customer AS. Since the customer only learns routes from the desired entry point, they will prefer that entry point for inbound traffic. This is a standard technique for influencing inbound traffic without requiring customer-side configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Set community 'no-export' on routes from the undesired entry point.
Why this is correct
Set community 'no-export' on routes from the undesired entry point. This prevents those routes from being advertised to the customer AS, so the customer only sees routes from the desired entry point, making them prefer that entry point. Correct.
- ✗
Set community 'prepend 3' on routes from the desired entry point.
Why it's wrong here
Set community 'prepend 3' on routes from the desired entry point. 'prepend 3' is not a valid BGP community; it is an AS path prepend action. Incorrect.
- ✗
Set community 'local-preference 200' on routes from the desired entry point.
Why it's wrong here
Set community 'local-preference 200' on routes from the desired entry point. There is no standard BGP community named 'local-preference'; local preference is an attribute set on the receiving router, not via a community. Incorrect.
- ✗
Set community 'no-advertise' on routes from the undesired entry point.
Why it's wrong here
Set community 'no-advertise' on routes from the undesired entry point. While this also prevents advertisement, it is more restrictive than necessary as it also stops iBGP propagation within the provider AS. 'no-export' is more commonly used for this purpose. Incorrect in comparison.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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