You guys are confusing a lot of settings. The first post confused the MSS/Window settings for the internal TCP stack for telnet/ftp/web connections to the router itself with PC settings. These settings are not used when communicating with remote sites.
You found the interface MTU settings alright but additionally there is a separate TCP MSS limit of 1400 despite MTU of interface is 1492 (maximum MSS is 1452). Note that this for TCP connections only and when using PPPoE.
It works like this: If the MSS option in TCP connection establishment packets is greater than the limit set router modifies it down to 1400. Router does it to both SYN and SYN-ACK packets so both sides think the other side has a lower MSS and adjusts its send packet size accordingly. In other words, router is fooling both ends to achieve smaller packet size. UDP or ICMP does not carry MSS in header so they are not bound by this operation.
You can adjust MSS limit via:
ip adj wanif0 1452
This command is only effective after PPPoE connected.
Upcoming firmware has "ip adjmss 1452" command which sets the default MSS and it can be added to autoexec.net to make it permanent.
Here is a menu 24.8 session, typing "ip adj wanif0" will tell you current MSS on WAN:
314> 314> ip adj wanif0 adjust TCP mss on wanif0 to 1400 314> 314> ip adj wanif0 1452 adjust TCP mss on wanif0 to 1452 314> 314> ip adj wanif0 adjust TCP mss on wanif0 to 1452 314>
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