MTU in MPLS can be an issue since each MPLS label imposed on a frame decreases the transported protocol's maximum size.


  • MTU of an interface defines the maximum Layer 3 packet size that can be transmitted.
  • Each labeled packet adds 4 bytes for each MPLS header. MPLS VPN uses two labes, 2*4 = 8 + 1500 = 1508 bytes.
  • The default MPLS MTU is 1500.
  • Knowing the max number of labels, you can set the MPLS MTU with the command mpls mtu (value). For example 2 labels should have a MTU of 1508 to support 1500 IP packets.
  • For packets that exceed the MTU, MPLS strips the labels, fragments packets and puts back labels for each fragment unless the IP Don't Fragement bit is set.

Baby Giant Frame

  • Frames that are slightly bigger than the allowed MTU size like 1508 are called Baby Giant Frames.
  • These could be configured using the mpls mtu override command.

Maximum Receive Unit

  • Maximum Receive Unit - Cisco's parameter that depending on the label operation keeps track of the largest MTU size. This value is tracked based on the pop, push, swap…etc..for each prefix or FEC.

Relevant Commands


MTU size depends on media. Ethernet is set to 1500 bytes.

R1(config-if)#        mtu (size in bytes)

mlps mtu

MTU of MPLS frames can be specified. It has to equal to or less than the interface's MTU. One exception to the rule is the override option is configured. The override allows to configure Baby Giants on interfaces that support MTU up to 1580.

R1(config-if)#        mpls mtu (size in bytes)
R1(config-if)#        mpls mtu override (Int's MTU - 1580)


rating: 0+x
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License