MTU is short for maximum transmission unit and plays an important role when two devices on the same network interconnect with each other. The MTU determines the maximum number of bytes that can be transmitted by a sender at a time. If the MTU exceeds the maximum number of bytes supported by the receiver or a transit device, packets are then fragmented or even discarded, which aggravates the network transmission load. To prevent the problem, devices have to calculate the MTU before communications so that sent packets can reach the receiver successfully.
LDP LSP forwarding and common IP forwarding differ greatly in terms of implementation mechanism but share a large number of similar aspects about the MTU. Both of them are required to send packets smoothly to the receiver through devices at each hop without reassembly.
The MPLS MTU, like the interface MTU, has a default value and is configurable. Before informing the upstream device of the LDP MTU, an LSR calculates the LDP MTU by selecting the smallest value among the MTU values used by all downstream devices and the MTU of the egress. The LSR puts the value of the smaller MTU in the MTU TLV of a Label Mapping message and then sends the message to the upstream device. If any of the two MTUs mentioned previously changes due to configuration modifications or the outbound interface changes on the local end, the LSR recalculates the MTU and sends a Label Mapping message that contains the calculated MTU to all upstream devices.