Specifications

mpls mtu
MP-274
Cisco IOS Multiprotocol Label Switching Command Reference
January 2010
Caution If you do not set the MPLS MTU less than or equal to the interface MTU, data corruption, dropped
packets, and high CPU conditions can occur.
• Changing the interface MTU can also modify the IP MTU, Connectionless Network Service (CLNS)
MTU, and other MTU values, if they depend on the value of the interface MTU. The Open Shortest
Path First (OSPF) routing protocol requires that the IP MTU values match on both ends of the link.
Similarly, the Intermediate System-to-Intermediate System (IS-IS) routing protocol requires that the
CLNS MTU values match on both ends of the link. If the values on both ends of the link do not
match, IS-IS or OSPF cannot complete its initialization.
General Usage Guidelines
• ATM interfaces cannot accommodate packets that exceed the Segmentation and Reassembly (SAR)
buffer size, because labels are added to the packet. The bytes argument refers to the number of bytes
in the packet before the addition of any labels. If each label is 4 bytes, the maximum value of bytes
on an ATM interface is the physical MTU minus 4*x bytes, where x is the number of labels expected
in the received packet.
• If a labeled IPv4 packet exceeds the MPLS MTU size for the interface, Cisco IOS software
fragments the packet. If a labeled non-IPv4 packet exceeds the MPLS MTU size, the packet is
dropped.
• All devices on a physical medium must have the same MPLS MTU value in order for MPLS to
interoperate.
• The MTU for labeled packets for an interface is determined as follows:
– If the mpls mtu bytes command has been used to configure an MPLS MTU, the MTU for
labeled packets is the bytes value.
– Otherwise, the MTU for labeled packets is the default MTU for the interface.
• Because labeling a packet makes it larger due to the label stack, you may want the MPLS MTU to
be larger than the interface MTU or IP MTU in order to prevent the fragmentation of labeled packets,
which would not be fragmented if they were unlabeled. In Cisco IOS Release 12.2(25)S and later
releases, the MPLS MTU cannot be larger than the interface MTU.
• Changing the interface MTU value (using the mtu interface configuration command) can affect the
MPLS MTU of the interface. If the MPLS MTU value is the same as the interface MTU value (this
is the default), and you change the interface MTU value, the MPLS MTU value will automatically
be set to this new MTU as well. However, the reverse is not true; changing the MPLS MTU value
has no effect on the interface MTU.
Examples The following example shows how to set the interface MTU value and MPLS MTU value for a serial
in
terface:
interface Serial4/0
mtu 1520
ip unnumbered Loopback0
mpls mtu 1510
mpls traffic-eng tunnels
mpls ip
serial restart-delay 0
ip rsvp bandwidth 2000 2000