The answer is that the SPAN destination port cannot be a trunk port; it must be an access port. This is because SPAN destination ports are designed to be dedicated monitoring ports that should not participate in normal network traffic, spanning tree, or MAC address learning. When Gi0/1 is configured as a trunk with native VLAN 1, the switch may forward mirrored traffic from VLAN 20 as tagged frames, which the analyzer may not interpret correctly, or the port may drop the mirrored traffic due to VLAN membership conflicts. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of SPAN restrictions and the common trap of assuming a trunk port can serve as a destination for local SPAN. The key is that a SPAN destination port strips all VLAN tags by default unless explicitly configured to preserve them, and it must be an access port to avoid interference from other VLAN traffic. Memory tip: SPAN destination ports are "single-purpose access ports"—no trunking, no learning, just mirroring.
300-410 SPAN, RSPAN, and ERSPAN Practice Question
This 300-410 practice question tests your understanding of span, rspan, and erspan. 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 network engineer configures SPAN on a switch to monitor traffic from VLAN 20 to a local analyzer on interface Gi0/1. The configuration: monitor session 1 source vlan 20 rx monitor session 1 destination interface Gi0/1. The analyzer sees no traffic. The switch runs OSPF and has a route to 0.0.0.0/0 via a neighbor. The Gi0/1 interface is configured as a trunk port with native VLAN 1. The analyzer is connected to a hub that also connects to another device. The other device sends traffic that causes the switch to learn MAC addresses on Gi0/1. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The destination interface Gi0/1 is a trunk port, which is not supported for SPAN destinations; it must be an access port.
SPAN destination ports should not be used for normal traffic; they are dedicated to monitoring. If the destination port is configured as a trunk, it may participate in spanning tree and learn MAC addresses. However, the key issue is that SPAN does not work if the destination port is a trunk port that is also used for other traffic. The switch may be forwarding traffic from other VLANs to the analyzer, but the mirrored traffic from VLAN 20 may be dropped because the destination port is not in the correct VLAN. Specifically, the destination port should be an access port in the same VLAN as the source, or if it is a trunk, the mirrored traffic is sent as untagged or tagged depending on the configuration. The analyzer may not receive traffic if the native VLAN mismatch occurs. The correct root cause is that the destination port is a trunk with native VLAN 1, but the mirrored traffic from VLAN 20 is tagged with VLAN 20, and the analyzer may not understand the tag. However, the more subtle issue is that the switch may not allow the destination port to be a trunk for SPAN; it must be an access port. The correct answer is that SPAN destination ports cannot be trunk ports.
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.
✓
The destination interface Gi0/1 is a trunk port, which is not supported for SPAN destinations; it must be an access port.
Why this is correct
SPAN destination ports must be access ports; trunk ports are not allowed as SPAN destinations.
The source VLAN 20 is not allowed on the trunk port Gi0/1.
Why it's wrong here
Even if allowed, trunk ports are not supported for SPAN destinations.
✗
The analyzer is connected via a hub, causing a loop that disables the port.
Why it's wrong here
The hub may cause a loop, but the scenario does not indicate a loop detection.
✗
The monitor session is missing the 'no shutdown' command.
Why it's wrong here
SPAN sessions are enabled by default.
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.
Trap categories for this question
Scenario analysis trap
The hub may cause a loop, but the scenario does not indicate a loop detection.
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.
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 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 300-410 OSPF questions on adjacency and route selection.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The destination interface Gi0/1 is a trunk port, which is not supported for SPAN destinations; it must be an access port. — SPAN destination ports should not be used for normal traffic; they are dedicated to monitoring. If the destination port is configured as a trunk, it may participate in spanning tree and learn MAC addresses. However, the key issue is that SPAN does not work if the destination port is a trunk port that is also used for other traffic. The switch may be forwarding traffic from other VLANs to the analyzer, but the mirrored traffic from VLAN 20 may be dropped because the destination port is not in the correct VLAN. Specifically, the destination port should be an access port in the same VLAN as the source, or if it is a trunk, the mirrored traffic is sent as untagged or tagged depending on the configuration. The analyzer may not receive traffic if the native VLAN mismatch occurs. The correct root cause is that the destination port is a trunk with native VLAN 1, but the mirrored traffic from VLAN 20 is tagged with VLAN 20, and the analyzer may not understand the tag. However, the more subtle issue is that the switch may not allow the destination port to be a trunk for SPAN; it must be an access port. The correct answer is that SPAN destination ports cannot be trunk ports.
What should I do if I get this 300-410 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 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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