The answer is that the ERSPAN destination IP is reachable via a route pointing to the tunnel interface itself, causing recursive routing. When ERSPAN monitors a tunnel interface, it copies the original IP packets before tunnel encapsulation, then adds a new ERSPAN header with the destination IP. If the routing table sends that ERSPAN-encapsulated traffic back into the same tunnel (e.g., via 10.0.0.2, the tunnel peer), the packet gets double-encapsulated and dropped, as the tunnel cannot handle its own mirrored output. This scenario tests your understanding of how ERSPAN interacts with overlay networks on the CCNP ENARSI 300-410 exam, often appearing as a tricky troubleshooting question where the source and destination IPs share the same tunnel path. A common trap is assuming the tunnel interface is transparent to mirrored traffic, but the key insight is that the ERSPAN process uses the global routing table, not the tunnel’s forwarding context. Memory tip: “Don’t send the mirror through the mirror—if the destination routes via the tunnel, you’ll double the trouble.”
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.
Router R1 is configured as an ERSPAN source to monitor traffic on interface Tunnel0 (a DMVPN tunnel). The destination is a collector at 172.16.0.100. R1's configuration: monitor session 1 type erspan-source source interface Tunnel0 both destination erspan-id 4 ip address 172.16.0.100 origin ip address 10.0.0.1. The collector receives no packets. The Tunnel0 interface is up/up and has an IP of 10.0.0.1/30. The tunnel destination is 192.168.1.1. The router has a route to 172.16.0.100 via 10.0.0.2. What is the root cause?
The ERSPAN source interface is a tunnel, which is not supported for ERSPAN.
Why wrong: ERSPAN can monitor tunnel interfaces.
B
The ERSPAN destination IP is routed via the tunnel interface, causing the encapsulated packets to be sent into the tunnel and double-encapsulated, leading to drop.
The route to 172.16.0.100 via 10.0.0.2 points to the tunnel peer, so the encapsulated packets enter the tunnel, causing recursion.
C
The tunnel interface is not in the same VRF as the collector.
Why wrong: No VRF is mentioned.
D
The ERSPAN session ID 4 conflicts with the tunnel key.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ERSPAN destination IP is routed via the tunnel interface, causing the encapsulated packets to be sent into the tunnel and double-encapsulated, leading to drop.
ERSPAN monitors traffic on the source interface. When the source is a tunnel interface, the mirrored traffic is the original IP packets before encapsulation. The ERSPAN process then encapsulates these packets with a new IP header. However, the encapsulated packets are sent using the routing table. If the destination IP is reachable via a route that points to the tunnel interface itself (e.g., via 10.0.0.2, which is the tunnel's peer), the encapsulated packets may be sent into the tunnel, causing them to be encapsulated again. This double encapsulation can cause issues. But the more likely root cause is that the ERSPAN session is configured to monitor the tunnel interface, but the tunnel interface itself may not be able to generate the encapsulated packets correctly because the tunnel is used for the destination. The correct answer is that the ERSPAN source IP is the same as the tunnel interface IP, and the destination is routed via the tunnel, causing a recursive routing loop similar to the first question.
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 ERSPAN source interface is a tunnel, which is not supported for ERSPAN.
Why it's wrong here
ERSPAN can monitor tunnel interfaces.
✓
The ERSPAN destination IP is routed via the tunnel interface, causing the encapsulated packets to be sent into the tunnel and double-encapsulated, leading to drop.
Why this is correct
The route to 172.16.0.100 via 10.0.0.2 points to the tunnel peer, so the encapsulated packets enter the tunnel, causing recursion.
Related concept
Static NAT maps one inside address to one outside address.
✗
The tunnel interface is not in the same VRF as the collector.
Why it's wrong here
No VRF is mentioned.
✗
The ERSPAN session ID 4 conflicts with the tunnel key.
Why it's wrong here
No conflict.
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.
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 ERSPAN destination IP is routed via the tunnel interface, causing the encapsulated packets to be sent into the tunnel and double-encapsulated, leading to drop. — ERSPAN monitors traffic on the source interface. When the source is a tunnel interface, the mirrored traffic is the original IP packets before encapsulation. The ERSPAN process then encapsulates these packets with a new IP header. However, the encapsulated packets are sent using the routing table. If the destination IP is reachable via a route that points to the tunnel interface itself (e.g., via 10.0.0.2, which is the tunnel's peer), the encapsulated packets may be sent into the tunnel, causing them to be encapsulated again. This double encapsulation can cause issues. But the more likely root cause is that the ERSPAN session is configured to monitor the tunnel interface, but the tunnel interface itself may not be able to generate the encapsulated packets correctly because the tunnel is used for the destination. The correct answer is that the ERSPAN source IP is the same as the tunnel interface IP, and the destination is routed via the tunnel, causing a recursive routing loop similar to the first question.
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.
About these practice questions
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Variation 1. 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?
hard
A.The monitor session is administratively down due to a missing 'no shutdown' command.
B.The ERSPAN destination IP address is in a different VRF that is not reachable from the source VRF.
✓ 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.
D.The ERSPAN session ID 100 conflicts with an existing GRE key on the tunnel.
Why C: 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.
Last reviewed: Jun 19, 2026
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