Question 482 of 2,152

SPAN, RSPAN, and ERSPAN →hardMultiple ChoiceObjective-mapped

ERSPAN Recursive Routing via Tunnel Interface

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?

A
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.
Why wrong: No conflict.

Full breakdown with real-world context →

Quick Answer

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.”

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 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: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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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 — Read the scenario before looking for a memorised answer..

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?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

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.

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