Example of TCP MSS Adjustment
Consider the example in the following figure. Host A is a TCP client, Host B is the TCP server, and there are two routers, A and B. Router A has TCP MSS adjustment configured on ingress port Ethernet 1/1/1 with a specific value because there is a GRE tunnel in the path, which will add 24 bytes of encapsulation. Router A modifies the TCP MSS adjustment value in the TCP SYN packet and forwards the packet. Host B receives the TCP SYN packet with the TCP MSS adjustment value set to the same value. Host B compares its MSS value to the received MSS value and sends the lower value to Host A as the "Send MSS" value. Host B send its own MSS value in the TCP SYN-ACK. In Router A, egress port Ethernet 1/1/1 is configured with the TCP MSS adjustment value. Router A modifies the MSS value in the TCP SYN-ACK and forwards the packet. Host A receives the TCP SYN-ACK and sends the "Send MSS" value to Host B.
Because Host A and Host B set a specific "Send MSS" value, neither of them will send a TCP data segment greater than a specific number of bytes to the other host. IP fragmentation of the TCP data packet is unnecessary even though GRE tunnel encapsulation occurs at Router A and Router B.
The same outcome can be achieved by configuring TCP MSS adjustment in the GRE tunnel interface in Router A. In this case, the MSS will be modified only for the packets traversing the tunnel. The remaining traffic will not be modified.