GRE MTU Calculator
GRE adds about 24 bytes to every packet, so over a normal 1500-byte link the inner MTU is 1476.
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Encapsulation in the Path
Inner MTU
1500 − 24 bytes of headers
TCP MSS (IPv4)
TCP MSS (IPv6)
Largest Ping (IPv4)
Clamp MSS (nftables)
nft add rule inet filter forward tcp flags syn tcp option maxseg size set 1436
MTU & MSS Calculator
GRE on a 1500-byte link
Overhead24 bytes
Inner MTU1476
TCP MSS (IPv4)1436
TCP MSS (IPv6)1416
Over PPPoE (1492)1468
Testping -M do -s 1448 <host>
If the inner interface keeps an MTU of 1500, packets larger than 1476 bytes are fragmented or dropped. The usual symptom: ping and small requests work, large downloads and TLS handshakes hang.
Set the GRE interface's MTU to 1476, or clamp TCP MSS to 1436 on the router.
MTU by Tunnel
Questions
What MTU should I use for GRE?
1476 over a standard 1500-byte link; 1468 over PPPoE.
How much overhead does GRE add?
About 24 bytes per packet.
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