MTU Problems: Troubleshooting Fragmentation and Black-Hole Paths
Recognize MTU mismatches, oversized frames and path-MTU discovery failures.
Recognize the strange symptoms MTU causes
MTU faults often allow small packets while breaking larger transactions. Pings may work while file transfers stall, VPN sessions fail, or particular web pages load only partly. That pattern should move MTU higher on the list.
Tunnels, MPLS labels, VLAN tags, encryption, and overlays add overhead. A payload that fit before encapsulation may no longer fit after extra headers are applied.
Test the real path with controlled sizes
Use packet-size tests with fragmentation disabled where policy and the operating system permit it. Start below the expected path limit and increase the size until it fails, accounting for whether the tool reports payload size or full packet size.
Run the test across the same tunnel, provider segment, or service path the application uses. A successful local test proves little if it avoids the failing encapsulation.
Fix the narrowest point and retest
Compare interface MTU, tunnel MTU, routed MTU, and relevant service settings on both ends. Also verify that required ICMP feedback for Path MTU Discovery is not being blocked by a firewall.
The repair may be an MTU correction, tunnel adjustment, permitted ICMP, or an appropriate TCP MSS setting. Repeat the same size test and the original application test after the change.