Tunnel MTU calculator
MTU for nested tunnels and multi-hop transit paths.
What it is for
Nested and hop scenarios use different math: nested stacks overheads on the same packet; hops size each tunnel on its own underlay (full hop capacity).
Why it helps
Upstream GRE can stay at 1456 while customer WireGuard is 1420; path MTU and MSS use the narrowest hop without cutting capacity on wider hops.
Calculation mode
Multiple encapsulations on the same packet with no intermediate decapsulation. Overheads are subtracted from the outer path MTU.
Presets
Default underlay MTU
Typically Ethernet 1500. In hops mode each hop can use its own underlay MTU.
Tunnels / hops
Top is closest to the physical path (e.g. WG over the Internet). Bottom is the overlay on it (e.g. VXLAN VTEPs on wg0). Pick peer/VTEP IPv4 or IPv6 per layer.
Results
If VLAN tags or unknown options are present, round down for safety (for example 1370 → 1360).
WireGuard, VXLAN, GRE, IPIP, VLAN, and PPPoE use documented sizes. IPsec, OpenVPN, and L2TP/IPsec are typical values (cipher and NAT-T dependent). EoIP follows MikroTik (~42 B).
Configure MTU is what to set on that hop’s interface: min(underlay − overhead, prior path). If an earlier hop is narrower, a later wider hop is clamped (e.g. after WG 1420 the next GRE becomes 1420, not 1456). Path so far is the running e2e limit.
| # | Layer / hop | Overhead | MTU after layer |
|---|