The mpls mtu command configures the maximum transmission unit (MTU) of MPLS packets on an interface.
The undo mpls mtu command restores the default configuration.
By default, the MTU of MPLS packets is equal to the interface MTU.
| Parameter | Description | Value |
|---|---|---|
| mtu | Specifies the MPLS MTU of an interface. | The value range varies according to the interface type. |
Ethernet interface view, Ethernet sub-interface view, Eth-Trunk interface view, Eth-Trunk sub-interface view, Tunnel interface view, Dialer interface view, VLANIF interface view, Virtual-Template interface view
Usage Scenario
This command cannot be used on Layer 2 interfaces.
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 LSP-based forwarding and IP-based forwarding are different in terms of implementation, but are similar in terms of MTU calculation. If MTU values are correctly negotiated between LSRs before packet transmission, packets can successfully reach the receiver without fragmentation and reassembly.
To calculate the MPLS MTU, an LSR on the path to a specified FEC compares all MTUs advertised by downstream devices with the interface MTU of its own, and adds the smaller MTU (the MPLS MTU) to the MTU TLV field in the Label Mapping message, and send the Label Mapping message upstream.
If an MTU value changes (such as 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.
If an interface MTU, 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 two MTUs is used.
Prerequisites
MPLS has been enabled on the interface using the mpls command.
Configuration Impact
After the mpls mtu command is run, LDP compares the MPLS MTU and the interface MTU, and uses the smaller value between the two MTUs.