The mpls mtu command configures the maximum transmission unit (MTU) value for MPLS packets on an interface.
The undo mpls mtu command restores the default MTU value.
By default, the MPLS MTU is equal to the interface MTU.
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Usage Scenario
An MTU value determines the maximum number of bytes of a packet each time a sender can send without fragmenting the packet. 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. The relationships between the MPLS MTU and the interface MTU are as follows: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.
Precautions
By default, when determining the length of an MPLS packet to be forwarded by an interface, a device calculates only the length of the IP packet encapsulated in the MPLS packet. If the interface-mtu check-mode command is run on the device with check-mode being label-contained-length, the device also counts in the length of the MPLS label.