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300-410 Practice Question: Router R1 is configured as an ERSPAN source to…
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?
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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