(Optional) Configuring an MPLS MTU on an Interface

An MPLS MTU can be configured on an interface to determine the maximum size of MPLS packets that can be forwarded each time.

Context

MTU is short for maximum transmission unit. An MTU value determines the maximum number of bytes each time a sender can send. If the size of packets exceeds the MTU supported by a transit node or a receiver, the transit node or receiver fragments the packets or even discards them, aggravating the network transmission load. MTU values must be correctly negotiated between LSRs to allow packets to successfully reach the receiver.

LDP MTU = Min { All MTUs advertised by downstream devices, Local outbound interface MTU }

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 upstream.

If an MTU value changes (such as when the local outbound interface or its configuration is changed), an LSR recalculates an MTU value and sends a Label Mapping message carrying the new MTU value upstream. The comparison process repeats to update MTU information along an LSP.

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.

Procedure

Access the system view.
```
system-view
```
Access the view of an MPLS-enabled interface.
```
interface interface-type interface-number
```
Configure an MPLS MTU on the interface.
```
mpls mtu mtu
```
The configured MPLS MTU takes effect immediately and there is no need to restart the interface.