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350-401 BGP Path Selection Practice Question
An enterprise is implementing MPLS L3VPN to connect multiple branch offices. The PE routers are using eBGP to exchange VPNv4 routes. The engineer notices that some VPN routes are not being advertised to the remote PE. The 'show bgp vpnv4 unicast all' on the local PE shows the routes as valid but not best. What is the most likely reason?
⚠ Common exam trap
Candidates often assume that a valid route is always advertised, but BGP only advertises the best path. The trap here is confusing local preference and MED: high local preference increases preference, while high MED decreases preference.
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 MED value than the best path.
In BGP, the route with the highest local preference is preferred, so option A is incorrect. The route is valid, so options B and C are incorrect because a missing label would make it invalid, and being in the BGP table is confirmed. MED (Multi-Exit Discriminator) is used in the BGP path selection process; a higher MED value makes a route less preferred compared to a lower MED. Therefore, if the route has a higher MED than the best path, it will not be selected as best, even though it is valid. This is the most likely reason for the route being valid but not best.
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 higher local preference than the best path.
Why it's wrong here
Local preference is a weight-like attribute evaluated very early in BGP best-path selection, where the highest local preference wins. If this route carried a higher LP than the current best path, BGP would elect it as the best path, not the other way around. A lower LP could explain a loss, but a higher one cannot. Therefore, this option states the opposite of what would actually happen.
- ✗
The route is not valid due to a missing label.
Why it's wrong here
A missing label would make a VPNv4 route invalid, but the BGP table output marks this route as valid. The 'valid' status indicates that the next hop is resolvable and, for MPLS VPN prefixes, a label is already assigned for the route. Therefore, the absence of a label cannot be the reason the route is not best; the route passed the validity check but lost on path attributes instead.
- ✗
The route is not in the BGP table.
Why it's wrong here
If the route were not in the BGP table, the 'show ip bgp' output would not list it at all. Since this specific prefix and path appear in the output, it is undeniably present in the BGP table. A route can be present yet not selected as the best path; that is a path-selection outcome, not a missing-table issue. Thus, this explanation contradicts the very evidence being analyzed.
- ✓
The route has a higher MED value than the best path.
Why this is correct
Multi-Exit Discriminator (MED) is an attribute carried across an AS and is compared only for routes from the same neighboring AS; the lowest MED is preferred. A higher MED value makes a route less desirable and would cause BGP to select the competing path with the lower MED as best. Therefore, this route being non-best is consistent with having a higher MED than the best path.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.