The difference between the MTU seen by end - nodes ( e.g. 1500 ) .
The Path MTU causes Path MTU Discovery to come into effect , with the possible result of making some sites behind badly configured firewalls unreachable .
The difference between the MTU seen by end - nodes ( e.g. 1500 ) and the Path MTU causes Path MTU Discovery to come into effect , with the possible result of making some sites behind badly configured firewalls unreachable .