Why do packets secretly vanish over VPN tunnels?
Troubleshooting MTU issues on VPN tunnels is a classic CCNA exam scenario. Drop your answer in the comments before the timer runs out!
The question
Why do large packets fail across an IPsec tunnel while small pings succeed?
The answer
A Tunnel overhead exceeds the link MTU
Tunnel headers add extra bytes to the packet, pushing it past the standard fifteen-hundred-byte MTU. Watch out on the CCNA: if the DF bit is set, the router drops it instead of fragmenting.
Exam tip
Read what the question is actually asking before looking at the options. The wrong answers here are ones candidates genuinely pick — each is true of something adjacent, just not of what was asked.