- A
The monitor session is administratively down due to a missing 'no shutdown' command.
Why wrong: The configuration shows 'no shutdown' is present.
- B
The ERSPAN destination IP address is in a different VRF that is not reachable from the source VRF.
Why wrong: There is no VRF configuration mentioned; the routing table shows a default route, implying global routing table.
- C
The GRE tunnel interface is used for the ERSPAN source IP, causing a recursive routing loop because the destination IP is routed via the tunnel's next-hop.
The source IP 10.1.1.1 is the tunnel interface IP, and the destination 192.168.10.10 is routed via 10.1.1.2, which is the tunnel's next-hop. This recursion causes the encapsulated packet to be dropped.
- D
The ERSPAN session ID 100 conflicts with an existing GRE key on the tunnel.
Why wrong: ERSPAN ID is a session identifier, not a GRE key; no conflict is indicated.
300-410 SPAN, RSPAN, and ERSPAN Practice Question
This 300-410 practice question tests your understanding of span, rspan, and erspan. 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 ERSPAN on Router R1 to monitor traffic from VLAN 100 to a remote collector at 192.168.10.10 via a GRE tunnel. The source interface is GigabitEthernet0/0/0. After configuration, the collector receives no mirrored packets. R1's configuration: monitor session 1 type erspan-source source interface Gi0/0/0 both destination erspan-id 100 ip address 192.168.10.10 origin ip address 10.1.1.1 no shutdown. R1's routing table shows a default route via 10.1.1.2, and a static route to 192.168.10.0/24 via 10.1.1.2. The tunnel interface Tunnel0 is up/up with IP 10.1.1.1/30. What is the most likely root cause?
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 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 GRE tunnel interface is used for the ERSPAN source IP, causing a recursive routing loop because the destination IP is routed via the tunnel's next-hop.
ERSPAN encapsulates mirrored packets in GRE with a destination IP of the collector. The router must have a route to the collector IP, but the encapsulated packets use the routing table of the default VRF. If the destination IP is reachable via a route that points to a next-hop that is not directly connected, the router may attempt to use the GRE tunnel interface itself, causing a recursive routing loop. The GRE tunnel interface IP (10.1.1.1) is used as the source, but the destination 192.168.10.10 is routed via 10.1.1.2, which is the tunnel's next-hop. This creates a recursion: the packet is encapsulated with destination 192.168.10.10, then routed, which again matches the tunnel, leading to a loop and packet drop. The fix is to use a separate source IP or ensure the route to the collector does not point back through the tunnel.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 monitor session is administratively down due to a missing 'no shutdown' command.
Why it's wrong here
The configuration shows 'no shutdown' is present.
- ✗
The ERSPAN destination IP address is in a different VRF that is not reachable from the source VRF.
Why it's wrong here
There is no VRF configuration mentioned; the routing table shows a default route, implying global routing table.
- ✓
The GRE tunnel interface is used for the ERSPAN source IP, causing a recursive routing loop because the destination IP is routed via the tunnel's next-hop.
Why this is correct
The source IP 10.1.1.1 is the tunnel interface IP, and the destination 192.168.10.10 is routed via 10.1.1.2, which is the tunnel's next-hop. This recursion causes the encapsulated packet to be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The ERSPAN session ID 100 conflicts with an existing GRE key on the tunnel.
Why it's wrong here
ERSPAN ID is a session identifier, not a GRE key; no conflict is indicated.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
The configuration shows 'no shutdown' is present.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 300-410 question test?
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The GRE tunnel interface is used for the ERSPAN source IP, causing a recursive routing loop because the destination IP is routed via the tunnel's next-hop. — ERSPAN encapsulates mirrored packets in GRE with a destination IP of the collector. The router must have a route to the collector IP, but the encapsulated packets use the routing table of the default VRF. If the destination IP is reachable via a route that points to a next-hop that is not directly connected, the router may attempt to use the GRE tunnel interface itself, causing a recursive routing loop. The GRE tunnel interface IP (10.1.1.1) is used as the source, but the destination 192.168.10.10 is routed via 10.1.1.2, which is the tunnel's next-hop. This creates a recursion: the packet is encapsulated with destination 192.168.10.10, then routed, which again matches the tunnel, leading to a loop and packet drop. The fix is to use a separate source IP or ensure the route to the collector does not point back through the tunnel.
What should I do if I get this 300-410 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 19, 2026
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