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300-410 Practice Question: An engineer configures BGP between two routers in…
An engineer configures BGP between two routers in the same AS. The iBGP session is established, but routes learned from the eBGP neighbor are not advertised to the iBGP peer. The 'show ip bgp' output shows the routes as valid but not best. Which is the most likely explanation?
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 IP of the eBGP route is not reachable via IGP, making the route invalid.
For iBGP, the next-hop must be reachable via IGP. If the next-hop is not reachable (e.g., no IGP route for the eBGP peer's interface IP), the route is considered invalid and not installed. This is a common edge case where next-hop-self is not configured.
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 'synchronization' rule is enabled and the IGP does not carry the prefix.
Why it's wrong here
Synchronisation requires the IGP to carry the iBGP-learned prefix before it is advertised to eBGP neighbours, but the problem here is that routes are not advertised to an iBGP peer, not that they are withheld from eBGP. This option is tempting because synchronisation does suppress route advertisement when the IGP lacks the prefix, which would match a scenario where an eBGP-learned route is missing from the IGP and thus not propagated to external peers.
- ✓
The next-hop IP of the eBGP route is not reachable via IGP, making the route invalid.
Why this is correct
iBGP requires next-hop reachability; without next-hop-self, the next-hop is the eBGP peer's IP.
- ✗
The 'maximum-paths' command is set to 1, preventing load balancing.
Why it's wrong here
Maximum-paths affects load balancing, not route advertisement.
- ✗
The 'bgp bestpath med missing-as-worst' command causes the route to be considered worse.
Why it's wrong here
This affects best path selection but not next-hop reachability.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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