CompTIA Network+ (N10-009)Network TroubleshootingMedium
A network administrator increases the MTU on a core router interface to 9000 bytes for jumbo frame support, but forgets to update an edge switch, which remains at the default 1500-byte MTU. Users report that small file downloads work fine, but large file transfers across this path consistently fail or hang. What is the MOST likely explanation?
- AAn MTU mismatch causing fragmentation or packet drops for large packets
- BA duplex mismatch between the router and switch
- CAn expired DHCP lease on the client's fragmentation router
- DDNS caching an outdated record for the file server
Show answer & explanationAnswer & explanation
Correct answer: A. An MTU mismatch causing fragmentation or packet drops for large packets
When devices along a path have mismatched MTU settings, packets larger than the smallest MTU in the path may be dropped (if fragmentation is disabled, such as with the Don't Fragment flag) or require fragmentation, which some paths mishandle, causing large transfers to hang while small packets pass fine.
Why the other options are wrong
- B. Duplex mismatch causes collisions and slow speeds regardless of packet size, not size-dependent failures.
- C. DHCP lease expiration is unrelated to packet size and would affect all traffic, not just large transfers.
- D. DNS caching would cause name resolution to an old IP, not size-dependent transfer failures.
MTU Mismatch
A condition where devices along a network path have differing Maximum Transmission Unit settings, causing large packets to be dropped or improperly fragmented.
- Default Ethernet MTU is 1500 bytes; jumbo frames use up to 9000 bytes
- Symptom: small packets succeed, large packets fail or hang ('black hole' MTU issue)
- Diagnosed with ping using specific packet sizes and the Don't Fragment flag
Memory trick: Big boxes get stuck at a small doorway — mismatched MTU blocks large packets.