350-501 Architecture Practice Question
A service provider is troubleshooting a BGP route advertisement issue. Routes from a customer are not being advertised to the upstream provider. The PE router is configured with 'neighbor 10.0.0.1 route-map RMAP out'. The route-map RMAP permits the customer prefix. However, the BGP table on the PE shows the prefix as valid but not advertised. What is a likely cause?
⚠ Common exam trap
Cisco often tests the subtle distinction between a prefix being 'valid' (next-hop reachable in the global routing table) and 'advertised' (next-hop reachable from the specific neighbor's perspective), leading candidates to overlook next-hop reachability as the root cause.
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 next-hop is not reachable from the upstream provider
The BGP table shows the prefix as valid but not advertised, which indicates that BGP has the route but is not sending it to the upstream neighbor. A common cause is that the next-hop for the customer prefix is not reachable from the PE router via the interface used to reach the upstream provider. BGP will not advertise a route if the next-hop is not reachable in the routing table (unless 'neighbor x.x.x.x next-hop-self' is configured), because the upstream router would be unable to forward traffic to that next-hop.
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 next-hop is not reachable from the upstream provider
Why this is correct
If next-hop-self is not used, the next-hop might be a customer-facing interface not reachable upstream.
- ✗
The BGP session to the upstream provider is flapping
Why it's wrong here
If flapping, the route might appear and disappear, but the scenario indicates stable.
- ✗
The prefix is not in the global routing table
Why it's wrong here
It is in the BGP table, so it is in the routing table.
- ✗
The route-map is applied inbound instead of outbound
Why it's wrong here
The configuration shows outbound, so that is not the issue.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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