I'm configuring ip tcp adjust-mss to 1200 on a router interface and when doing a packet trace I still see the MSS value showing up as 1460 in the intial SYN packet.
The setup is as follows; I have a pc connected to a router on Fa0/0. I have placed the command 'ip tcp adjust-mss 1200' in interface Fa0/0 configuration. This router then connects to another router. I issue a telnet from the host pc to the remote router, so the SYN is going through the router I have lowered the MSS value on and it just isn't changing. I'm using wireshark to verify. Am I missing something or is this command supposed to tell the router to intercept SYN packets and change the MSS value?
Any help is much appreciated.
The SYN ACK with MSS as 536 is expected. Most platforms use 536 as the default MSS. During TCP 3 way handshakes, whichever side has smaller size MSS will be used.