The answer is that the route has a higher local preference than the best path, which prevents it from being selected as the best path and thus from being advertised to remote PEs. In BGP, only the single best path for a prefix is installed in the routing table and propagated to neighbors; a route marked as valid but not best has passed all validation checks (like next-hop reachability) but lost the BGP path selection tie-breaker, most commonly due to a higher local preference value on the winning route. This scenario directly tests your understanding of BGP path selection order on the ENCOR 350-401 exam, where local preference is evaluated before AS-path length, MED, and IGP metric. A common trap is assuming a valid route will always be advertised, but BGP’s fundamental rule is that only the best path is shared. Memory tip: think of local preference as the “team captain” that overrides all other attributes—if your route has a lower LP, it sits on the bench as valid but never plays.
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 than the best path.
In BGP, only the best path is advertised to peers. If a route is valid but not best, it may be due to a higher local preference or other BGP path selection criteria. Option A is correct. Option B is wrong because the route is valid; Option C is wrong because the route is present; Option D is wrong because MED is not typically set by default.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 this is correct
Correct because BGP selects the best path based on local preference; a higher local preference makes a route less preferred.
Clue confirmation
The clue words "best", "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The route is not valid due to a missing label.
Why it's wrong here
Incorrect because the route is listed as valid.
✗
The route is not in the BGP table.
Why it's wrong here
Incorrect because the route is present.
✗
The route has a higher MED value than the best path.
Why it's wrong here
Incorrect because MED is only compared for routes from the same AS.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
MPLS — This question tests MPLS — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The route has a higher local preference than the best path. — In BGP, only the best path is advertised to peers. If a route is valid but not best, it may be due to a higher local preference or other BGP path selection criteria. Option A is correct. Option B is wrong because the route is valid; Option C is wrong because the route is present; Option D is wrong because MED is not typically set by default.
What should I do if I get this 350-401 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "best", "most likely". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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