hardMultiple Choice
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 wraps the original frame in a GRE header plus a new IP header, enlarging each packet beyond the original frame size. If this exceeds the path MTU and fragmentation is blocked, the collector receives truncated or malformed payloads.
- ✗
The collector is not configured to reassemble IP fragments.
Why it's wrong here
ERSPAN encapsulates mirrored frames in GRE, and large frames exceed the path MTU, producing fragmentation that the collector must reassemble. It is tempting because fragmentation handling is a genuine requirement, and it would be correct when captured packets exceed the tunnel MTU.
- ✗
The ERSPAN session is configured with the wrong source interface, causing incorrect IP headers.
Why it's wrong here
A wrong source interface changes which packets are mirrored, not the payload's integrity; the collector would still parse intact ERSPAN headers. It tempts because source-interface selection genuinely matters when traffic must egress a specific path or when the collector expects packets from a particular address. Here, truncation or malformed payloads point to MTU or jumbo-frame mismatch instead.
- ✗
The router has IPsec configured on the egress interface, encrypting the ERSPAN packets.
Why it's wrong here
IPsec encryption preserves the ERSPAN packet's payload integrity, so it cannot cause truncation or malformation; the collector would simply fail to decrypt without matching SAs. This option is tempting because IPsec does alter packets on egress, and MTU overhead from encapsulation can fragment traffic — but that scenario concerns tunnel interfaces, not ERSPAN payload corruption.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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