The maximum transmission unit (MTU) defines the maximum number of bytes that a device can transmit at a time. It plays an important role when two devices communicate on a network. If the MTU exceeds the maximum number of bytes supported by a receive end or a transit device, packets are fragmented or even discarded, which imposes heavy burden on network transmission. Devices must calculate MTUs before communicating so that the sent packets can successfully reach receive ends.
LDP LSP forwarding differs greatly from IP forwarding in terms of implementation mechanisms, but they share a large number of similarities regarding the MTU principles. Both the LDP MTU and IP MTU are used so that packets pass through each transit device smoothly and reach receivers without fragmentation or reassembly.
An LSR selects the smallest value as the LDP MTU among MTUs advertised by all preferred next hops and the MTU of the local outbound interface. The LSR then sends upstream LSRs through Label Mapping messages carrying the MTU TLV representing the calculated LDP MTU. If any MTU changes due to the local outbound interface change or configuration change, the LSR recalculates the MTU and sends Label Mapping messages carrying the calculated MTU to all upstream LSRs.