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300-410 Practice Question: An engineer configures ERSPAN on a Cisco router…
An engineer configures ERSPAN on a Cisco router to send mirrored traffic to a remote collector via IP. The collector receives the ERSPAN packets, but the payload appears truncated or malformed. What is the most likely 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 GRE encapsulation adds overhead exceeding the path MTU, causing fragmentation or drop.
ERSPAN encapsulates mirrored packets in GRE with an additional ERSPAN header. The MTU of the path between the source and collector must accommodate the extra overhead (typically 50 bytes for GRE + ERSPAN). If the path MTU is too small, packets are fragmented or dropped, causing truncation.
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 GRE encapsulation adds overhead exceeding the path MTU, causing fragmentation or drop.
Why this is correct
ERSPAN adds 50 bytes of overhead; if the original packet is near MTU, the encapsulated packet exceeds it, leading to fragmentation or loss.
- ✗
The collector is not configured to reassemble IP fragments.
Why it's wrong here
While possible, the primary issue is MTU; fragmentation itself is not inherently malforming.
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The ERSPAN session is configured with the wrong source interface, causing incorrect IP headers.
Why it's wrong here
Wrong source interface would affect reachability, not payload integrity.
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The router has IPsec configured on the egress interface, encrypting the ERSPAN packets.
Why it's wrong here
IPsec would encrypt, not truncate; the collector would see encrypted data, not malformed payload.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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