Firewall, DSL, PPPoE and Path MTU discovery

When configure DSL broadband setup, PPPoE mode is usually the option to choose, not the bridge mode, which do not require username/password and broadcast or share the link with your neighbour. PPPoE will bring 28 byte or more overhead to Ethernet fram underneath, thus the MTU size on the source need to reduce ( 1500 -28) to avoide the fragment along the way.
I remembered when I worked for Truestar 4 years ago, there are many times I need to reduce the MTU size to as low as 1460 because BELL support ask me to do so.
More often than not, there is a Path MTU discovery method built-in to automatically do this for you to automatically detect the smallest MTU size for all intermediate devices. However, many firewall discard the ICMP, which cause the discovery failed.

http://en.wikipedia.org/wiki/Path_MTU_discovery