MTU is the Maximum Transmission Unit of general frames entering/exiting the interface
IP MTU is the MTU for IP packets entering/exiting the interface.
The difference is there because of the different features you can add (ie. run IPX etc.)
If you have MTU-problems, try setting "ip tcp adjust-mss 536" on you inside interface... you could always adjust to a bigger value.. use a ping with different sizes to see which size gets through..
If you use the microsoft-ping, remember that the "-l size" parameter (-s for unix-version) is the payload of the ICMP-packets, so you should subtract 28 bytes (20 bytes IP header + 8 bytes ICMP header) from the total.. If you ping with a size of 1472, this would equal a IP-packet with MTU of 1500.
The MSS is the Maximum Segment Size, ie the data payload of TCP-packets.. this should be (normally in a ethernet-environment) 1500 - 20 bytes IP header - 20 bytes TCP header = 1460 bytes. The minimum IP MTU which must be supported on the 'net is 576 bytes, which gives a MSS of 536.
If you are running PPPoE, remember to take this into account... Check this link for more info:
http://www.cisco.com/en/US/tech/tk175/tk15/technologies_tech_note09186a0080093bc7.shtml
