- A
BGP synchronization is enabled, and the prefix is not present in the IGP.
Correct. Synchronization prevents route installation if IGP does not have the prefix.
- B
The next-hop-self command is missing on the iBGP peer.
Why wrong: Missing next-hop-self would cause unreachable next-hop, but next-hop is reachable.
- C
The prefix is filtered by an inbound route-map.
Why wrong: Filtered prefix would not appear in BGP table.
- D
The maximum-paths limit is exceeded.
Why wrong: Maximum-paths affects load balancing, not installation.
Question 765 of 2,152
iBGP Synchronization Preventing Route Installation
This 300-410 practice question tests your understanding of nat and pat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 engineer configures iBGP between two routers in the same AS. The BGP table shows the prefix, but it is not installed in the routing table. The next-hop is reachable via an IGP route. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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.
Quick Answer
The answer is BGP synchronization being enabled and the prefix not present in the IGP. This is correct because when BGP synchronization is active, an iBGP route will not be installed into the routing table unless the same prefix is also learned via an Interior Gateway Protocol (IGP), even if the next-hop is reachable. The synchronization rule exists to prevent black-holing traffic when routers in the transit path do not run iBGP, ensuring the IGP has full knowledge of the prefix before it is used. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how iBGP route installation differs from eBGP, and it often appears as a trap where candidates mistakenly blame next-hop unreachability. A common memory tip is “sync stops the sink”—if synchronization is on, the route sinks out of the routing table until the IGP confirms it.
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
✓
BGP synchronization is enabled, and the prefix is not present in the IGP.
Option A is correct because when BGP synchronization is enabled, the router will not install an iBGP-learned prefix into the routing table unless the same prefix is also present in the IGP (e.g., OSPF or EIGRP). Since the next-hop is reachable via IGP but the prefix itself is not in the IGP, the synchronization rule blocks the route from being installed. This is a classic scenario where the BGP table shows the prefix, but it is missing from the routing table.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
BGP synchronization is enabled, and the prefix is not present in the IGP.
Why this is correct
Correct. Synchronization prevents route installation if IGP does not have the prefix.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The next-hop-self command is missing on the iBGP peer.
Why it's wrong here
Missing next-hop-self would cause unreachable next-hop, but next-hop is reachable.
- ✗
The prefix is filtered by an inbound route-map.
Why it's wrong here
Filtered prefix would not appear in BGP table.
- ✗
The maximum-paths limit is exceeded.
Why it's wrong here
Maximum-paths affects load balancing, not installation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the BGP synchronization rule as a subtle cause of routes being in the BGP table but not in the routing table, leading candidates to mistakenly focus on next-hop reachability or filtering issues.
Detailed technical explanation
How to think about this question
BGP synchronization (RFC 1771, now deprecated in modern Cisco IOS) requires that an iBGP-learned prefix must also be known via an IGP before it is considered valid for installation. This rule was designed to prevent black-holing when routers in the transit path do not run BGP. In real-world networks, synchronization is typically disabled (no synchronization) to allow iBGP routes to be installed immediately, as long as the next-hop is reachable.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: BGP synchronization is enabled, and the prefix is not present in the IGP. — Option A is correct because when BGP synchronization is enabled, the router will not install an iBGP-learned prefix into the routing table unless the same prefix is also present in the IGP (e.g., OSPF or EIGRP). Since the next-hop is reachable via IGP but the prefix itself is not in the IGP, the synchronization rule blocks the route from being installed. This is a classic scenario where the BGP table shows the prefix, but it is missing from the routing table.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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