FAQ-What is the Relationship Between the MTU Value of an S12700 Interface and the MPLS MTU Value of an Interface

A: The relationship between the MTU value of the interface and the MPLS MTU of the interface is as follows:

1. If no MPLS MTU value is configured on an interface, the interface MTU is used to control forwarding of  MPLS packets.  
2. If both an MPLS MTU value and an interface MTU value are configured on an interface, the smaller value between the two MTU values is used to control forwarding of MPLS packets.

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 outbound interface. A downstream LSR uses the preceding formula to calculate an MTU value, 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 (for example, when the local outbound interface or its configuration changes), an LSR recalculates the MTU value and sends a Label Mapping message carrying the new MTU value to all upstream devices.