The answer is a configuration checksum mismatch between the two BGP peers. This is the most likely cause of BGP flapping after a FortiOS upgrade because the new firmware can introduce default changes or altered behaviors in BGP update messages, such as new optional transitive attributes or modified path attributes, which cause the checksum computed by the local FortiGate to differ from what the service provider peer expects. When the checksums do not match, the peer rejects the update, causing the session to drop and cycle through the Active state, as seen in the 'diagnose ip router bgp all' output. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your understanding of post-upgrade BGP stability issues and the importance of verifying peer configurations for consistency, with a common trap being to blame MTU or TTL mismatches instead. Remember the memory tip: "Checksum clash, session crash" — always compare the BGP configuration checksums between peers after a firmware upgrade to isolate flapping.
NSE7 Troubleshooting and Diagnostics Practice Question
This NSE7 practice question tests your understanding of troubleshooting and diagnostics. 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.
A FortiGate administrator notices that after upgrading the firmware, some BGP sessions to a service provider are flapping. The administrator runs 'diagnose ip router bgp all' and sees that the BGP neighbor state is Active. What is the MOST likely cause of this issue?
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
✓
A configuration checksum mismatch between the two peers
Option B is correct because a configuration checksum mismatch between peers is a common post-upgrade issue that causes BGP sessions to flap. A new default setting or changed behavior in the new firmware can alter the update message, leading to a checksum mismatch.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
A configuration checksum mismatch between the two peers
Why this is correct
Firmware upgrades can introduce new defaults that change the update message, causing checksum mismatch and flapping.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Hold timer expiry would result in the session going to Idle, not Active.
✗
The BGP neighbor is not reachable due to a routing issue
Why it's wrong here
If the neighbor were unreachable, the state would be Idle, not Active.
✗
The BGP password is incorrect
Why it's wrong here
An incorrect password would cause the session to stay in Idle state.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related NSE7 OSPF questions on adjacency and route selection.
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: A configuration checksum mismatch between the two peers — Option B is correct because a configuration checksum mismatch between peers is a common post-upgrade issue that causes BGP sessions to flap. A new default setting or changed behavior in the new firmware can alter the update message, leading to a checksum mismatch.
What should I do if I get this NSE7 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related NSE7 OSPF questions on adjacency and route 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?
OSPF neighbours must agree on key parameters.
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