Configuring MPLS MTU
MPLS inserts the label stack between the link layer header and network layer header of each packet. To make sure the size of MPLS labeled packets is smaller than the MTU of an interface, configure an MPLS MTU on the interface.
MPLS compares each MPLS packet against the interface MPLS MTU. When the packet exceeds the MPLS MTU:
If fragmentation is allowed, MPLS removes the label stack from the packet, fragments the IP packet (the length of a fragment is the MPLS MTU minus the length of the label stack), adds the label stack to each fragment, and forwards the fragments.
If fragmentation is not allowed, the LSR drops the packet.
To configure an MPLS MTU for an interface:
Step | Command | Remarks |
|---|---|---|
1. Enter system view. | system-view | N/A |
2. Enter interface view. | interface interface-type interface-number | N/A |
3. Configure an MPLS MTU for the interface. | mpls mtu value | By default, no MPLS MTU is configured on an interface. |
The following applies when an interface handles MPLS packets:
MPLS packets carrying L2VPN or IPv6 packets are always forwarded by an interface, even if the length of the MPLS packets exceeds the MPLS MTU of the interface. Whether the forwarding can succeed depends on the actual forwarding capacity of the interface.
If the MPLS MTU of an interface is greater than the MTU of the interface, data forwarding might fail on the interface.
If you do not configure the MPLS MTU of an interface, fragmentation of MPLS packets is based on the MTU of the interface without considering MPLS labels. An MPLS fragment might be larger than the interface MTU and be dropped.