// Tool
MTU / MSS Calculator
Calculate effective payload, MSS, and packet size after tunnel encapsulation overhead. Stack multiple encapsulations to model nested designs (VXLAN over IPsec, GRE over IPsec, etc.). Built-in scenario presets for common topologies. Get ready-to-paste adjust-mss commands for Cisco IOS, NX-OS, Juniper, and Linux.
Frequently Asked Questions
What is the difference between MTU and MSS?
MTU is the largest layer-3 packet an interface will send, including IP and TCP headers. MSS is the largest TCP payload — MTU minus the IP and TCP headers (normally 40 bytes for IPv4).
Why do tunnels require lowering MTU or MSS?
Each encapsulation (GRE, IPsec, VXLAN, and so on) adds header bytes. If the inner packet plus overhead exceeds the path MTU, it fragments or is dropped — so you lower the inner MTU/MSS to fit.
What is MSS clamping?
A technique where the router rewrites the MSS value in TCP SYN packets to a size that fits the tunnel, so endpoints negotiate a payload that fits without relying on Path MTU Discovery.
