Has it ever crossed your mind that your just updated antivirus is not doing it’s job? We all think that we are protected but how do we even know that we are? Ok, enough with the rhetorical questions and let’s move to the point.
MTU size never crossed my mind until MPLS designed network came to the picture. After adding a switch to the network, OSPF got up, but BGP was going up and down and up and down… That led to the MTU mismatch because bigger frames are needed in a MPLS network.