CCNP Infrastructure Practice Question
A multinational organization has a BGP-based MPLS VPN network. The CE router at a branch office is connected to two PE routers (PE1 and PE2) in the service provider network. The branch uses eBGP to exchange routes with the PEs. The network administrator notices that the branch can reach some destinations but not others. The BGP table on the CE shows routes with next-hop set to the PE loopback addresses, but those loopbacks are not reachable. The CE has a default route pointing to the PEs. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the BGP next-hop reachability rule, where candidates mistakenly think a default route satisfies the next-hop check, but BGP requires a specific route to the next-hop address (not a default route) for the route to be installed in the routing table.
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 addresses of the BGP routes are not reachable.
The CE router learns BGP routes from the PE routers with next-hop addresses set to the PE loopback interfaces. For these routes to be installed in the routing table, the CE must have a route to the next-hop IP address. Since the CE only has a default route pointing to the PEs and the PE loopbacks are not directly connected or reachable via any specific route, the BGP routes remain hidden (not installed) because the next-hop is unreachable. This is the most likely cause of partial reachability.
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 addresses of the BGP routes are not reachable.
Why this is correct
In BGP, the route is only installed into the IP routing table if the next-hop IP address is reachable via an existing IGP or static route. In an MPLS VPN, the PE advertises VPN routes to the remote PE with the next-hop set to the loopback address of the advertising PE. If that loopback is not in the IGP routing table of the receiving PE or the CE, the BGP route remains in the BGP table but is not installed, causing the CE to see no usable routes.
- ✗
The default route on the CE is overriding the BGP routes.
Why it's wrong here
A default route (0.0.0.0/0) on the CE has the least specific prefix length, so it is used only for destinations that do not match a more specific route. BGP-advertised prefixes (e.g., /24s) are more specific and will always be preferred over the default route for matching traffic, regardless of the administrative distance. Thus, a default route cannot override or mask BGP routes; it simply provides a fallback when no specific route exists.
- ✗
The routes have an AS path that is too long.
Why it's wrong here
AS path length is a BGP best-path selection criterion, not a route-installation filter. BGP accepts and installs any valid route from a peer, even if its AS path is very long, as long as it passes basic validation (e.g., next-hop reachability). In practice, an AS path would have to exceed the 255-attribute segment limit to be considered invalid, which is extremely rare and not a typical reason for missing routes.
- ✗
The CE is not advertising its routes to the PEs.
Why it's wrong here
This explanation confuses outbound route advertisement with inbound route reception. The CE not advertising its own routes to the PEs would affect what the PE learns and can redistribute, but it does not prevent the CE from receiving BGP routes that the PE sends. The issue described is that the CE is not receiving/installing routes from the PE, which is a problem with the BGP session or the route attributes (like next-hop reachability), not with what the CE advertises.
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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.