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MTU / MSS & Fragmentation
Stack encapsulation overheads (VLAN, GRE, IPsec, PPPoE, VXLAN…) onto a base MTU to see the effective inner MTU, the resulting TCP MSS, whether full-size packets fragment, and the ip tcp adjust-mss value that fixes the classic VPN/tunnel black-hole.
base MTU (bytes)
inner L3
encapsulation (stackable)
Total overhead
80 B
Effective MTU
1420 B
TCP MSS (v4)
1380 B
1500-B packets
fragment ⚠
1500 − GRE 24 − IPsec ESP (tunnel) 56 = 1420 B available to the inner IPv4 packet; MSS = effective MTU − 20 IP − 20 TCP.