MTU & Fragmentation Calculator

Enter your interface MTU and select tunnel/encapsulation overheads. See usable payload, whether fragmentation will occur, and the exact MSS clamp value to fix it.

Physical interface MTU (bytes)

Test packet size (bytes, optional)

Leave at 1500 to test standard Ethernet packets

// Select encapsulations / tunnel overhead

Custom overhead (bytes)

Custom label

Interface MTU

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Total Overhead

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Usable Payload

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Recommended MSS

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Fragmentation

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PMTUD Path MTU

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Packet structure visualisation

// Verification commands

Test path MTU with ping DF-bit:

ping 8.8.8.8 df-bit size 1472 repeat 5

Check interface MTU:

show interface GigabitEthernet0/0 | include MTU

Check TCP MSS adjustment:

show running-config | include tcp adjust-mss

// why MTU matters

MTU (Maximum Transmission Unit) is the largest packet that can traverse a network link. When a packet exceeds the MTU, it must be fragmented — split into smaller pieces and reassembled at the destination. Fragmentation causes performance problems, and many networks block ICMP, breaking PMTUD (Path MTU Discovery).

Tunnel overhead is the biggest cause of MTU issues. GRE adds 24 bytes. IPsec adds 50–73 bytes. MPLS adds 4 bytes per label. Stack these together and your usable payload drops significantly below 1500.

MSS clamping is the fix. The MSS (Maximum Segment Size) in TCP SYN packets tells the other end the maximum payload per segment. By clamping MSS on the tunnel interface, you prevent TCP from sending segments too large to traverse the path without fragmentation.