This 300-410 practice question tests your understanding of bgp troubleshooting. 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.
R1 and R2 are eBGP peers. R1 advertises a prefix 192.168.1.0/24 with MED 50. R2 also receives the same prefix from another eBGP peer R3 with MED 100. R2's BGP best path selection chooses the path via R1 because of lower MED. However, R2's routing table shows the next-hop for 192.168.1.0/24 as 10.1.1.1 (R1), but R2 cannot ping 192.168.1.1. R2 has: interface GigabitEthernet0/0, ip address 10.1.1.2 255.255.255.0, and no ip route to 192.168.1.0/24 other than BGP. R1's interface to R2 has ip address 10.1.1.1 255.255.255.0. R1's BGP table shows the prefix with next-hop 10.1.1.1 (self). What is the root cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
R1 does not have the prefix 192.168.1.0/24 in its routing table because the next-hop for that prefix in R1's BGP table is unreachable.
R1's BGP table may have the prefix, but if the next-hop is not reachable (e.g., due to missing route), R1 will not install it in the routing table, causing forwarding failure.
B
R2's MED comparison is incorrect; lower MED should be preferred, but R2 should have chosen R3.
Why wrong: Lower MED is preferred, so R2 correctly chose R1.
C
R2 is missing a static route to 192.168.1.0/24.
Why wrong: BGP provides the route; the issue is on R1.
D
R1 should use next-hop-self when advertising to R2.
Why wrong: R1 already uses its own IP as next-hop; that is not the issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R1 does not have the prefix 192.168.1.0/24 in its routing table because the next-hop for that prefix in R1's BGP table is unreachable.
The issue is that R1 is advertising the prefix with next-hop 10.1.1.1, which is the interface IP of R1 towards R2. R2 installs the route with that next-hop, and since it is directly connected, R2 can reach it. However, R1 may not have a route to 192.168.1.0/24 in its routing table (e.g., it learned it via iBGP but did not install it due to next-hop unreachable or other issue). R1's BGP table shows the prefix, but if the next-hop is not reachable in R1's routing table, R1 will not install it in the routing table, and thus cannot forward packets. The root cause is that R1 does not have a valid route to the next-hop of the prefix it is advertising, so it cannot forward traffic.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
R1 does not have the prefix 192.168.1.0/24 in its routing table because the next-hop for that prefix in R1's BGP table is unreachable.
Why this is correct
R1's BGP table may have the prefix, but if the next-hop is not reachable (e.g., due to missing route), R1 will not install it in the routing table, causing forwarding failure.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
R2's MED comparison is incorrect; lower MED should be preferred, but R2 should have chosen R3.
R1 should use next-hop-self when advertising to R2.
Why it's wrong here
R1 already uses its own IP as next-hop; that is not the issue.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
BGP Troubleshooting — This question tests BGP Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: R1 does not have the prefix 192.168.1.0/24 in its routing table because the next-hop for that prefix in R1's BGP table is unreachable. — The issue is that R1 is advertising the prefix with next-hop 10.1.1.1, which is the interface IP of R1 towards R2. R2 installs the route with that next-hop, and since it is directly connected, R2 can reach it. However, R1 may not have a route to 192.168.1.0/24 in its routing table (e.g., it learned it via iBGP but did not install it due to next-hop unreachable or other issue). R1's BGP table shows the prefix, but if the next-hop is not reachable in R1's routing table, R1 will not install it in the routing table, and thus cannot forward packets. The root cause is that R1 does not have a valid route to the next-hop of the prefix it is advertising, so it cannot forward traffic.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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