NSE4 System and Network Administration Practice Question
An administrator configures an aggregate interface (port1 and port2) on a FortiGate. After connecting the switch ports, the aggregate interface shows 'down'. The individual member ports are up. What is the MOST likely cause?
⚠ Common exam trap
Candidates often assume the aggregate interface will come up automatically if the member ports are physically up, overlooking the requirement for matching aggregation configuration on the switch side.
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 switch ports are not configured for LACP or static aggregation
The aggregate interface remains down because the switch ports are not configured for LACP or static aggregation. For a FortiGate aggregate interface to come up, both the FortiGate member ports and the corresponding switch ports must be configured with the same aggregation protocol (LACP active/passive or static). Without this, the switch treats the ports as individual links, causing a mismatch that keeps the aggregate interface down.
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 member ports are set to different speeds
Why it's wrong here
The FortiGate allows member ports with different link speeds in the same aggregate interface, and the bundle will operate at the speed of the slowest member. This configuration affects throughput but does not prevent the aggregate from coming up, because the physical link is still active and LACP negotiation can still complete. Therefore, a speed mismatch between members is not the reason the interface is down.
- ✓
The switch ports are not configured for LACP or static aggregation
Why this is correct
An aggregate interface on a FortiGate requires the connected switch ports to be placed in a matching port-channel or LACP group. If the switch ports are left as ordinary access or trunk ports without LACP or static aggregation, the FortiGate does not receive the expected LACP protocol data units, and the aggregate interface remains down because the link-aggregation handshake never completes. This is the most common cause of an aggregate that is administratively up but physically down.
- ✗
The aggregate interface IP address is in the same subnet as the management interface
Why it's wrong here
Assigning an IP address to the aggregate interface that overlaps the subnet of the management interface creates a routing and ARP conflict, not a link-state problem. The aggregate interface can still reach the up state and forward traffic based on the routing table, but traffic destined to addresses in the overlapping subnet may be sent to the wrong interface. Consequently, this does not explain why the aggregate link is down.
- ✗
The FortiGate needs a reboot after creating an aggregate interface
Why it's wrong here
FortiGate applies aggregate interface configuration at runtime, so creating an aggregate and adding member ports does not require a reboot. The physical links and the aggregate bundle initialize immediately when the configuration changes are committed, regardless of whether the switch peer is correctly configured. Rebooting the FortiGate would not help and could actually mask the real issue of missing switch-side aggregation.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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