I keep running into issues with MTU and TCP MTU, yet I'm not following why it causes so many problems
ie. For mpls, you need to decrease the MTU to account for the MPLS labels.
ie.. For tunnel encapsulation (ipsec, GRE, etc), you need to decrease the mtu to account fro the additional overhead..
ie...BGP..if the mtu somewhere along the path is lower than what is negotiated, the peering will fail
The logic makes sense on why to lower the mtu to prevent fragmentation.
However, it would seem if these settings weren't set, the router/L3 device would just fragment. Other than the extra processing power to reassemble, it would still seem like fragmentation wouldn't cause so many connectivity issues.
If most hosts sent packets with the DF bit set, then I could easily see why fragmentation would cause issues. Most packets dont have the df bit set though so fragmentation shouldnt cause an issue.