hi@user_3423639
"The size of the maximum transmission unit (MTU) determines the maximum number of bytes that can be transmitted by the sender at a time. If the MTU exceeds the maximum number of bytes supported by the receiver or a transit device, packets are fragmented or even discarded, which increases the network transmission load. In this manner, devices have to calculate the MTU before the communication to ensure that sent packets reach the receiver successfully.
LDP MTU = Min {All MTUs advertised by all downstream devices, MTU of the local outbound interface}
A downstream LSR uses the preceding formula to calculate an MTU, adds it to the MTU TLV in a Label Mapping message, and sends the Label Mapping message to the upstream device. If an MTU value changes (such as when the local outbound interface or its configuration is changed), an LSR recalculates an MTU and sends a Label Mapping message carrying the new MTU to its upstream LSR. The relationships between the MPLS MTU and the interface MTU are as follows:
If an interface MTU but not an MPLS MTU is configured on an interface, the interface MTU is used.
If both an MPLS MTU and an interface MTU are configured on an interface, the smaller value between the MPLS MTU and the interface MTU is used.
MPLS determines the size of MPLS packets on the ingress node according to the LDP MTU to prevent the transit node from forwarding large-sized MPLS packets."
