(Optional) Configuring MPLS MTU

Context

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.

Procedure

Run system-view
The system view is displayed.
Run mpls ldp
The MPLS LDP view is displayed.
Run the following commands as required.
- Run undo mtu-signalling
  
  The LSR is disabled from sending Label Mapping messages carrying MTU TLVs.
  
  By default, the switch with MPLS LDP globally enabled sends Label Mapping messages carrying the MTU TLV, in compliance with draft-ietf-mpls-ldp-mtu-extensions.
  
  If a non-Huawei device does not support the MTU TLV, to implement interworking, configure the device not to encapsulate the MTU TLV in Label Mapping messages. If the LSR is disabled from sending the MTU TLV, the configured MPLS MTU does not take effect.
- Run mtu-signalling apply-tlv
  
  The LSR is configured to send Label Mapping messages carrying MTU TLVs that comply with RFC 3988.
  
  By default, the switch with MPLS LDP globally enabled sends Label Mapping messages carrying the MTU TLV, in compliance with draft-ietf-mpls-ldp-mtu-extensions.
  
  If a non-Huawei device supports the MTU TLV, to implement interworking, configure the device to send Label Mapping messages carrying MTU TLVs that comply with RFC 3988. Otherwise, the configured MPLS MTU may not take effect.
Enabling or disabling the function to send an MTU TLV leads the reestablishment of existing LDP sessions, resulting in MPLS service interruption.
Run quit
The system view is displayed.
Run interface interface-type interface-number
The view of an MPLS-enabled interface is displayed.
(Optional) On an Ethernet interface, run undo portswitch
The interface is switched to Layer 3 mode.

By default, an Ethernet interface works in Layer 2 mode.

Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support switching between Layer 2 and Layer 3 modes.
Run mpls mtu mtu
An MPLS MTU is configured on the interface.

By default, the MTU of MPLS packets is equal to the interface MTU.