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300-410 Practice Question: Configures SPAN on a switch to monitor traffic…
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 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 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.
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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