easyMultiple Choice
300-410 Practice Question: Is troubleshooting a BGP route advertisement issue
A network engineer is troubleshooting a BGP route advertisement issue. Router R1 (AS 65001) is an eBGP peer of R2 (AS 65002). R1 is advertising the prefix 10.0.0.0/8 to R2. R2 has an iBGP session with R3 (AS 65002). R3's BGP table shows the prefix 10.0.0.0/8 with next-hop 10.1.1.1 (R1's interface). However, R3 does not install this route in its routing table. The output of 'show ip route 10.0.0.0' on R3 shows no route. The engineer checks the routing table on R3 and sees that the interface connected to 10.1.1.0/24 is down. What is the most likely cause?
⚠ Common exam trap
The trap is assuming that BGP route installation depends on administrative distance or synchronization, when the most common cause is next-hop reachability. Candidates must check the next-hop reachability first.
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 10.1.1.1 is not reachable because the directly connected interface is down.
R3 does not install the route because the next-hop 10.1.1.1 is not reachable; the interface connected to 10.1.1.0/24 is down. BGP requires a valid next-hop to install a route in the routing table. Since the directly connected interface is down, the next-hop is unreachable, and the route is not installed.
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 10.1.1.1 is not reachable because the directly connected interface is down.
Why this is correct
iBGP does not rewrite next-hop by default, so R3 retains 10.1.1.1. BGP validates the next-hop against the routing table before bestowing best-path status; with the connected interface down, that address is unresolvable and the route stays unusable.
- ✗
The route is not installed because BGP synchronization is enabled.
Why it's wrong here
Synchronisation governs iBGP-to-IGP consistency, not next-hop reachability. R3's interface to 10.1.1.0/24 being down makes the eBGP-learned next hop unresolvable, so the route fails the next-hop check. Synchronisation would be the suspect only if the next hop were reachable via an IGP.
- ✗
The route is not installed because the prefix is being filtered by an inbound route-map on R3.
Why it's wrong here
An inbound route-map would remove the prefix from R3's BGP table entirely, yet the table still shows 10.0.0.0/8. The interface to 10.1.1.0/24 being down makes the next hop unreachable, so BGP cannot install the route. Route-maps are the right suspect when the prefix is absent from the table.
- ✗
The route is not installed because the administrative distance of the route is too high.
Why it's wrong here
Administrative distance decides between competing sources for the same prefix, not whether a single BGP route is usable. The down interface to 10.1.1.0/24 leaves the next hop unresolvable, so the route fails the next-hop validation before distance is considered. Distance matters when an IGP and BGP offer the same prefix.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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