The answer is that the ACL on GigabitEthernet0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied. ERSPAN works by taking a copy of the monitored traffic, then wrapping it in a new IP header using the origin IP address as the source; this newly encapsulated packet is then routed toward the collector. The critical technical concept here is that the ACL applied to the source interface filters the original traffic entering that interface, but if the router uses the same interface to forward the encapsulated packets outbound, the ACL will also inspect and potentially drop those packets. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how ERSPAN encapsulation interacts with interface ACLs—a common trap is assuming the ACL only affects the mirrored traffic, not the generated ERSPAN headers. Remember the memory tip: “ACLs see the packet as it leaves, not just as it arrives; if the origin IP is denied, the mirror is denied.”
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.
Router R1 is configured as an ERSPAN source to monitor traffic on interface GigabitEthernet0/0/0. The destination is a collector at 10.10.10.10. R1's configuration: monitor session 1 type erspan-source source interface Gi0/0/0 both destination erspan-id 3 ip address 10.10.10.10 origin ip address 192.168.1.1. The router has an ACL applied to the interface Gi0/0/0 that denies all IP traffic from source 192.168.1.1. The collector receives no packets. What is the root cause?
The ACL on Gi0/0/0 blocks the original traffic, so there is no traffic to mirror.
Why wrong: The symptom is that the collector receives no packets, but the original traffic may still be present; the ACL might block it, but the question is about the mirroring process.
B
The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.
If the encapsulated packets are sent out Gi0/0/0, the ACL will block them because the source IP is 192.168.1.1, which is denied.
C
The ERSPAN session is missing the 'no shutdown' command.
Why wrong: The configuration shows no 'shutdown' command, so it is enabled.
D
The destination IP 10.10.10.10 is not reachable.
Why wrong: The scenario does not indicate unreachability.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.
ERSPAN encapsulates the mirrored packets with a new IP header that has the source IP address specified in the 'origin ip address' command. The encapsulated packet is then sent out of the router using the routing table. However, the ACL on the source interface Gi0/0/0 is applied to the original traffic entering the interface, not to the encapsulated packets. The encapsulated packets are generated by the router and are not subject to the ACL on the source interface. The ACL would affect the original traffic being mirrored, but the symptom is that the collector receives no packets. The correct root cause is that the ACL is blocking the original traffic, so there is nothing to mirror. But the scenario says the collector receives no packets, meaning the mirrored packets are not being sent. The more likely issue is that the ACL is applied to the outgoing direction on the interface that sends the encapsulated packets, but the scenario says the ACL is applied to Gi0/0/0, which is the source interface. The encapsulated packets are sent out of a different interface (the one that has the route to the collector). The ACL on Gi0/0/0 does not affect the encapsulated packets. However, if the router uses the same interface to send the encapsulated packets, the ACL might block them. The correct answer is that the ACL is applied to the source interface and may block the encapsulated packets if they are sent out that interface.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 ACL on Gi0/0/0 blocks the original traffic, so there is no traffic to mirror.
Why it's wrong here
The symptom is that the collector receives no packets, but the original traffic may still be present; the ACL might block it, but the question is about the mirroring process.
✓
The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.
Why this is correct
If the encapsulated packets are sent out Gi0/0/0, the ACL will block them because the source IP is 192.168.1.1, which is denied.
Related concept
Static NAT maps one inside address to one outside address.
✗
The ERSPAN session is missing the 'no shutdown' command.
Why it's wrong here
The configuration shows no 'shutdown' command, so it is enabled.
✗
The destination IP 10.10.10.10 is not reachable.
Why it's wrong here
The scenario does not indicate unreachability.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
The configuration shows no 'shutdown' command, so it is enabled.
Scenario analysis trap
The scenario does not indicate unreachability.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied. — ERSPAN encapsulates the mirrored packets with a new IP header that has the source IP address specified in the 'origin ip address' command. The encapsulated packet is then sent out of the router using the routing table. However, the ACL on the source interface Gi0/0/0 is applied to the original traffic entering the interface, not to the encapsulated packets. The encapsulated packets are generated by the router and are not subject to the ACL on the source interface. The ACL would affect the original traffic being mirrored, but the symptom is that the collector receives no packets. The correct root cause is that the ACL is blocking the original traffic, so there is nothing to mirror. But the scenario says the collector receives no packets, meaning the mirrored packets are not being sent. The more likely issue is that the ACL is applied to the outgoing direction on the interface that sends the encapsulated packets, but the scenario says the ACL is applied to Gi0/0/0, which is the source interface. The encapsulated packets are sent out of a different interface (the one that has the route to the collector). The ACL on Gi0/0/0 does not affect the encapsulated packets. However, if the router uses the same interface to send the encapsulated packets, the ACL might block them. The correct answer is that the ACL is applied to the source interface and may block the encapsulated packets if they are sent out that interface.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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