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.
300-410 NAT and PAT Practice Question
This 300-410 practice question tests your understanding of nat and pat. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
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.
In iBGP, by default, the next-hop is not changed when advertising to iBGP peers (next-hop-self is not set). If the next-hop is not reachable via an IGP route (e.g., because the IGP does not carry the connected subnet of the eBGP peer), the route is not installed. However, the scenario says the next-hop is reachable, so another common issue is the BGP synchronization rule (when enabled) requiring the prefix to be present in the IGP before installing it.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
NAT and PAT — This question tests NAT and PAT — CIDR notation defines the prefix length..
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. — In iBGP, by default, the next-hop is not changed when advertising to iBGP peers (next-hop-self is not set). If the next-hop is not reachable via an IGP route (e.g., because the IGP does not carry the connected subnet of the eBGP peer), the route is not installed. However, the scenario says the next-hop is reachable, so another common issue is the BGP synchronization rule (when enabled) requiring the prefix to be present in the IGP before installing it.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 300-410 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 300-410 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.